The absolute basics:
A pro won't win a competition on a Wal-Mart bike.
An amateur won't win the a competition on a Tour de France bike.
Your bike should match your skill and your usage. If you want to do distance, you don't want a Mountain Bike. If you want to do trails, you don't want a BMX bike... so on and so forth.
If you have not decided yet, think about what type of riding would you like to do?
Vigorous exercise for 30min-1 hour?
Consistant exercise for 1-3 hours?
Varying exercise for 3-6 hours?
Once you figure out what you want to do, then the next step is to figure out how to do it. There is a wealth of information in these forums regarding each type of cycling.
Also, no matter how athletic you are, changing over to cycling is going to be a hard start. The more athletic you are, the easier the transistion will be, but be ready to find out about muscles you didn't know you had being in pain for a few days.
"It doesn't get any easier, you only get faster" - Greg Lemond
Be sure to check out the Training and Nutrition forum for a few tips on keeping yourself in shape and properly hydrated while cycling.
Don't expect criticism for asking too many questions. Most of us here love hearing about new cyclists coming to the scene and are happy to help