General Scenario Guide

General

The first step in a new park is to pause the game and have a look on your park. You can set up a few things as long as the game is paused and get a general overview.
Many scenarios already have prebuilt paths. Be sure to block off unused paths so your guests don't get lost. Avoid long walks and dead ends, so your guests don't have to wander around aimlessly.
Also check for Path Items to make sure there are enough benches (every 3-5 tiles) and litter bins (every 10-12 tiles).
You need enough Staff to keep your park running and clean. Set patrol areas to coordinate your handymen, while mechanics are usually good to run around freely (unless your park is kind of mazy).
A handymen can keep about 30-40 tiles of path clean, depending on how nauseating the rides next to them are are. Be sure to keep your park clean from litter and vomit as much as possible, otherwise guests will vandalize. If your park is kept clean, you don't need security guards at all.
Each mechanic can keep 3-4 rides running, keep adding as soon as they are fully stressed. You will have to reduce maintenance times once rides get older, so check your mechanics' status now and then.
Build enough stalls so your guests don't have to search long when they get hungry, thirsty or need to visit a bathroom. Bathrooms are also welcome if guests are very sick, so consider placing some of them near your most nauseating rides.
Place food stalls near less nauseating rides to prevent your guests from vomiting all over the area. Stalls with souveniers should go to your most exciting rides, since happy guests are more inclined to buy balloons etc.
Keeping your guests happy is always first priority, a higher park rating attracts more guests.

Research

You can setup Research as long as the game is paused, so do that as one of the first steps after beginning a scenario. Usually you start with a low variety of rides to build, so consider researching with maximum funds at the beginning. You can reduce that once you feel your park is doing well.

Most important is to fulfill the basic needs of your guests, that means you should research Shops and Stalls until Bathroom, Drink Stall (or similar), Information Kiosk, A.T.M.(RCT2) and at least one Food stall are available.
Next consider researching roller coasters until you have a good design available. Examples for good roller coasters are Steel Roller Coaster, Steel Corkscrew Roller Coaster, Wooden Crazy Rodent Roller Coaster and Steel Wild Mouse Roller Coaster since they allow to build cheap, compact and popular rides.
Since your guests have different needs, consider building a variety of Gentle Rides, Thrill Rides and roller coasters and set up research accordingly.
Researching Theming and ride upgrades isn't that important usually.

Finances

Money isn't usually that much of a problem in RCT, but still there are some common guidelines which help being succesful on a scenario.

As soon as you have a gate fee, even $1, your guests will pay less for your rides. The higher the gate fee, the less you can charge for your rides. Be sure to adapt to that and lower the prices of rides which have empty queues.
Bathrooms should always be free, it's just not worth it charging for them.
A guest thinking "... is really good value" means you can increase the price of the ride if you want to, but can also help you getting the Best-Value-Award.
As a rule of thumb, you can raise the price for coasters up until the excitement rating, but you have to discount shorter coasters. For tracked rides, the standard fee is usually ok. Keep an eye on ride admissions if you have a gate fee, since they are set to free by default then.
Your rides will get stale over time, so you have to drop the ride fees continually to keep attendance high. You might even have to offer a ride for free after some time or have to replace it. Some rides get stale quicker than others, like Scrambled Eggs or 3D Cinema.
The amount of money guests have can vary between scenarios, so keep an eye on that. In a scenario, this amount also varies by $30. Make sure that even the most poor guests can afford your gate fee and still ride some rides afterwards, otherwise they will leave very unhappy.
As soon as a guest is out of money, he will usually leave your park, unless you provide free rides, food, drinks and bathrooms for him. If you need a high number of guests, earning as much money as possible can contradict to the scenario goal.
You should always spend all your cash and maxime your loan! More rides attract more guests and usually every ride earns far more than it costs, even when including the (tiny!) interest rate. Keep building as long as you can afford and have room.
Before starting larger projects, you should provide a basic park with shops/stalls and a few rides.
If a scenario starts with one or more very large roller coasters or other tracked rides, consider demolishing it and building a few cheaper and smaller ones instead.
If you should ever run out of money and don't have an income to cover that, consider selling a ride or two. This shouldn't happen, though.

Creating good rides

Roller Coasters are the bread and butter of any park, so building good coasters and other custom rides is very important. The Ride Ratings show how well you designed a ride.

While most scenarios are about squeezing your rides in ungentle terrain, consider preparing some compact rides and save the track design to use in scenarios. You will usually find a spot for those rides and it gives you a head start in scenarios. Consider checking the Ride Exchange for track designs you can use.
Rely on Powered Launch to create compact coasters with a high capacity.
Queue Lines shouldn't be too long or too short. Guests will start complaining after a while and will finally leave after about 11 minutes in a queue. Entertainers can help with that, but consider also that waiting guests don't spend money. On the other hand, the line should be long enough to fill at least a complete train/all seats on a flat ride, otherwise the boarding process might take too long.
Try to interlock your queue line with the ride to increase its excitement rating. For details, check this link: Using Paths to Increase a Coaster's Excitement Rating
Scenery increases the excitement of your rides when placed 5 tiles around the "yellow arrow", this also counts for tracked rides (for multi-station rides, this works only on station 1). Check this link for details: What makes an Exciting Flat Ride?
Consider building underground when possible. Tunnels increase the excitement of any tracked ride, for some more than for others. Note that the number of tunnel entrances/exits counts for excitement and not the lengh of a tunnel. Any tracked ride which is more than 40% underground counts as indoor. Having some Indoor Rides is generally a good idea, so your guests can hide from the rain. Finally, you can conserve space by building underground.
Your guests have different needs you need to cater, so consider building rides with mixed intensity ratings.
If a ride offers more than one operation mode (like 3D Cinema, Whoa Belly or Gravitron), you can build multiple versions of this ride and have them operate differently. Your guests won't get bored of this.
To prevent a Crash from station brake failure, have the cars of a roller coaster enter the station at no more than 45 km/h (28 mp/h). Note that the brakes on track can also break down, so keeping a mechanic close every potentially dangerous roller coaster is advised.

Attracting and keeping more guests

If you are missing guests for the scenario objective, there are several measures you should consider using:

Marketing helps a lot and you should consider doing that from the 2nd to the last year. This will bring in more guests and depending on if they can afford being in your park that long, they will stay until the end of the scenario.
Lowering admission and ride fees increases the time guests can spend in your park before running out of money. With enough free rides and free stalls, guests might even not leave at all if your park can keep them happy. Also lower ride fees increase the satisfaction with your rides. Altogether, pricing can have a drastic effect on your guest count.
Building new rides makes people curious to visit your park.
Having a high park rating raises mouth-to-mouth propaganda for your park, that is basically free marketing. The happier your guests leave your park, the better the word they will spread.
Getting awards also attracts new guests and some of them are quite easy to get. Consider putting additional effort in getting awards and prevent getting bad awards.
As a last measure you can block the exit with a sign or by removing a path tile. Note that your park rating will drop very fast if guests are trapped in your park! Still this can help if it gets tight at the end of a scenario.