Two songs that are not at the same speed are difficult to mash up. In fact, in order to play 2 songs simultaneously, they must be at precisely the same tempo. Imagine two trains, running side by side. You can't jump from one to another if one is going at 135mph and the other at 85. You could fall and hurt yourself.
First, determine the song's BPM by visiting the Space Pirate BPM Calculator [here].
There are many other options for determining the tempo, but this one is always available if you're online.
It's noted that the BPM field in iTunes is not easily manipulated. But there's another category called "Genre" which is of little to no use for a Space Pirate. I've switched many of the entries in my iTunes "Genre" categories over to the tempo, such that I can browse many of my songs, after I've determined their tempo, like this:
Just as you're able to alphabetize your songs by "album" or by "artist," you can also organize your music by tempo. Just remember that there are many computer programs which claim to be able to determine the BPM for you, but it's never as good as "tapping out" the tempo manually.
Once you have organized some songs as tempo, you will find that many of them are similar. Often the ones that are similar in tempo really don't have anything to do with one another in genre. That's what makes it fun to combine them, which will be discussed in the next few steps.