You have basically two choices in getting started with your game:
1. Pull out the crayons and start drawing game levels, if you are wanting to get started quickly, you will need to make it a side scroller like the template game: Platformer, which shows up when you do File-New Project and it has an image that looks like Indiana Jones if he was Amish, who are a group of people with a culture that are much more interesting to me than the movies. I find that studying cultures like the Amish are a good idea if you are thinking about building games. Every religion has interesting ways of viewing the universe, and the Amish are no different. Ok, I digress…
2. Pull out a legal pad, One Note or open your Word Processor and start writing the design specification. Take a look at this Game Specification Format, it was written in 1997, but as a format I think that it is fine. Feel free to tune it up, let me know!
3. Use Popfly as a simple prototyping tool, it gives you and opportunity to play test your idea. There are other sites that work like Popfly, but it is pretty cool.
So you can do item 1 or 2, but item 3 using Popfly is more fun, but not for all people. It is interesting that great movie makers like Alfred Hitchcock felt that storyboarding (similar to the game specification) was more fun than the actual movie making. If you get a chance to read a biography about Alfred Hitchcock you will find out how a creative genius worked. This can be applied to the creation of games. Got to think big, and Alfred Hitchcock was large not only in body, but also in his body of work.
You might want to take crayon in hand get started on your game, right now. Use a notebook with a binder.