I'm somewhat new to all this, but my friend wanted me to make a tribute video based on characters from a tv show just for fun. I have an incomplete outline of what I want to do in the video, but I'm not exactly sure how to go about doing it.
I want to take a clip of one character, essentially remove the background so that it's just him moving, and then behind him have a series/montage of other scenes of him. Basically, the character itself would be moving independently of the other scenes in the background, yet still all simultaneously moving/doing something (if that makes sense).
I am aware of AE's rotoscoping capabilities to the extent that the background can be removed in order to isolate the character. However, I'm not sure of what to do afterwards with the character. Does AE have everything necessary to complete my task or do I need something like Adobe Premier alongside it? Do I have to create the "series/montage" beforehand in order to incorporate it as essentially the "background"?
For this video, a song will be playing over it.
You are overthinking this. Once you have isolated your talent you can do whatever you want with it either by using the result directly or rendering it out with transparency. While not an editing app, AE can handle short music videos reasonably well based on sequencing layers and adding effects. the rest will unfold as you work, but by all means, watch some tutorials. No offense, but you don't seem to have the slightest glimmer of what AE actually is and how it works and that's not a good starting point.
As Mylenium already said - first of all you need some more knowledge of how AE works and how to use it so watch some free tutorials, maby a paid course on lynda or somewhere with basics - after that you should be able to figure all out.