are you using a Mac or a PC ?

Garageband ships with newer macs and works well for what it is.

For the PC there are many options (some free demo software) and their are many threads in this forum that talk about the pros and cons of each.

As far as adaptors goes, you want a mono 1/4' to 1/8" adaptor. If you plug a stomp box or a guitar into a mic input you will overload it. This will not only sound bad but also make recording clean guitar very difficult. So, line level although not perfect is defiantly the way to go !