There are usually multiple strategies to successfully completing a scenario; these can be discussed on this article's talk page.
You should start by building a Wooden Crazy Rodent Roller Coaster, as it's fairly easy to get an excitement rating of 6.00, plus, it's fairly cheap as well. Then, you should construct some less intense rides and some shops. Once that's done, charge money for the rides and then charge an entrance fee. When you've got some more cash, build another roller coaster. Keep doing this until you have 10 coasters of at least a 6 Excitement rating. Don't be afraid to build the gentle/thrill rides, or coasters to meet the excitement early on just to get a quick profit.