If you are recording an instrument directly through the interface, you will need an amp simulator to get anything beyond an unprocessed clean tone. If you don't want to do this, then you will need a microphone to put in front of the amp and plug into the interface.
There are a lot of different interfaces designed for a lot of different things. Depending on what you want to do, what you look for varies a lot, and you are going to need either a microphone or an amp simulator to go with it.
What are you looking to do with it? Quickly put down ideas? Record tracks solo? Record your band?
What price range are you looking at?
Do you currently use/prefer any audio recording/editing software? Do you have any experience recording/mixing/editing?
What gear do you have right now? Specifically, microphones/amp/mixer/any other audio gear. What instruments are you recording?
What type of music are you recording?