You should learn basic music theory. I'm not sure what you mean by the pitch of the presets. But find out what key the acapella is in, and produce a track in the same key.
Mixing is very broad. There's lots of informations for free online. You should look into Equalizing, compression, and time effects such as delay and reverb.
But first is first. If you do not know basic music theory, or what a scale is. Then that would be your first priority before anything else.
