If you want to input your guitar then fortunately you can use just about any 'high impedence' input. If it were a microphone then you would be starting to run into quality issues.
are you using your mac or are you on an iPad? If you're on an iPad then Apogee's one is great, which is about $100 on the apple site (that's why I ask).
If you are on a computer it won't matter what interface you use, it will work on any program.
Also, what are you planning to do with it is an important question. If your'e looking at recording I would highly recommend getting a proper recording program and cutting out the 'baby step' which is most of the low end packages. Ultimately in the end you just lose time having to re-learn another program. There's a good article on the three main ones here: http://www.realguitarclass.com/which-daw-should-i-use/
If you're using it for just doing software distortion and stuff you can use anything. Great ones are Guitar Rig and Amplitube. Hope this helped a little bit I know it was a late repsonse!
I want to use garage band by inputing my guitar (electric)-I see Apple sells an interface for the ususal $99 but plenty of $30 ones on internet. Anyone know if the regular ones work? Can the Apple one work with Guitar 6 or such third party quitar programs?