How to mix music with software?

I want to mess around with music software just for fun. Specifically I would like to mash-up multiple songs like Girl Talk or Norwegian Recycling.

I downloaded Audacity and the FL Studio demo. I can’t figure out FL Studio, and Audacity won’t let me upload more than one track or even edit/add effects to the track that I did upload. The option to “Add New Audio Track” is light gray, meaning I cannot click on it.

So, do you guys have either any suggestions of software to try or tips for using Audacity or FL Studio?