Hi compositing is when you merge one or more layers together it's a bit like photoshop for video it can get very advanced....To the point where some places will charge you £700.00 PER HOUR

(This is feature film rates of course).
Lets say you have just filmed your video and now want to show it to someone so your going to need a title as your video starts so if you have "Red Squirrels Video" as a title over your first bit of video
that is VERY basic compositing.
In fact it's that basic most editors have that sort of thing built in these days so you don't need any extra compositing packages.
So to be honest for wildlife videos you don't need to worry about compositing.
Right I know nothing about you Red Squirrel but I bet you have used word on the computer and maybe excel? Correct?
If you have used these packages you can edit in Premeire Elements when ever you open up a new software package it all looks very scary but it really isn't
The three basic elements of edtitng are
1) Import your footage
2) Edit it
3) Render out the final cut (version)
And thats it

There are some cracking books out there to learn any software package the best one's I've found are the "How to do everything with" but the class room books are good as well.
Also youtube is a good source as well E.G
YouTube - Premiere Elements 9
If you PM me I'll give you my direct email address, other than that it's just a case of importing your footage and having a play.
Computer on and your book at the side of you