i know how to record, but i dont know how to get the backing track with my recording, I'm currently using reaper and a m-audio fast usb

I am not an expert, but it kind of goes like this.

"Track"->"Insert New Track" and then "Insert"->"Media File" such as .mp3 or .wav

You must download the backing track from the site, import it to the reaper with Insert Media File option, and create another track where you'll record your guitar playing. After that you must export mp3 file of your guitar along with the backing, and then solo the track where you recorded you guitar and export it also. Then upload both versions on the site through a post.