Not knowing where you’re starting from, you may want to look over this PDF from Adobe
https://www.adobe.com/designcenter/aftereffects/articles/aft65sd3dtrick/aft65sd3dtrick.pdf
I’ve used this kind of trick over and over!
Some tips, from me.
Reasonably small camera moves can really sell this without making the 2.5D really obvious.
If you decide where your camera is going to move (I duplicate a layer (or several) in Photoshop, then bring it in to AE and make those decisions, then update later) you don’t have to fill in all of the background… you fill in what’s going to be revealed!
Don’t neglect another tip that was stated, that it doesn’t have to be the original background, if you can find one that matches the feel, or know enough photoshop to make it feel right.
If you’re not that hot on Photoshop, I’d learn some about masks. I think it’s easiest to leave the original file intact, with masked layers as a PSD to adjust later if necessary.
I’m sure there are other, probably better tutorials than the link I gave. Andrew Kramer does a GREAT job of teaching some of the AE details of 3D
https://library.creativecow.net/articles/kramer_andrew/3dcompositing.php
and I learn easier watching than reading anyway…
Good luck!
Rhett