I'd also recommend that you take a look at some "non-amp" options, such as BOSS's JS10 or their BR80 Micro, or Korg's "Pandora"... these units offer built in effects that actually sound quite good, you can play with headphones (you can on a lot of amps too) and you can jam to backing tracks which I think is one of the best learning tools you can have.
Something that might be a good option for you right now is the Korg PDX 4 that you can buy used on e-bay for usually less than $100. A lot of amps and effect units feature a USB connection so you can plug the sound into your computer. However, the Korg (along with all the other units I mentioned) simply has a 1/8th jack so you can plug the headphone output of your computer into it. I find this is a much simpler method of jamming to tracks, going through lessons, etc. The USB into your computer is good if you want to record your guitar though.