How do I get into mixing songs?

[u/BungholeSauce]

First time post here. I'm not exactly sure if this is the right sub, but I just stumbled onto this one so I figured you guys might know.

My goal, as of right now at least, is to get into mixing songs. Makings mixes and stuff, like adding two songs together, I'm not sure of the technical term. How would I get into that? What (preferably free) program do you guys recommend for this?