Unless you live in one of the handful of states without a state income tax, you'll probably also have to file a state return. Depending on the state, the return could be more complex than your federal return, since most states have their own allowable credits and deductions.
When you do your taxes online, tax software like 1040.com Online Tax Preparation can simplify data entry for state returns by copying over the information you entered for your federal return, so you only need to fill out forms unique to that state. And, if you need to prepare more than one state return, 1040.com is unique in that one fee provides as many states as you need.
If you need more help, our Forms and Publications section provides downloadable state forms and instructions. We also provide a handy list of state tax sites. The state tax site is especially helpful for tracking your state refund, which can be done online for many states.