One of the best things about using an Android phone is that it's darn near impossible (OK, at least pretty difficult) to lose your data. That's not to say it can't happen, but Google's certainly given us some great tools to make it less likely.
When you first set up your phone, you'll be asked if you want to back up your account onto Google's servers, and if you want to automatically restore that data when you reinstall an app. Those options are also in the "Backup and reset" section of the Ice Cream Sandwich settings.
This is also where you'll find the option to reset -- or "hard reset" or "factory reset" -- your phone. This erases everything and returns it to the state it was when you first pulled it from the box.