"A bit familiar with photoshop" is like saying "I'm a bit familiar with astronomy"; it can mean different things to different people, and in your case I don't know whether the more appropriate interpretation is "I've seen it on the shelf in my local computer store", or "I've did a little image processing in a basic computer class", or "I can conceive of some object, and using photoshop, prepare a photorealistic illustration from scratch."
Search these forums for tutorials on using Photshop to reskin models, and do an internet search using your favorite search engine using the parameters "Trainz", "skin" "assets" and "Photoshop", and you'll probably find tutorials that would help you get started.
I would note that if you look at the download station contents, a surprising amount of the material on the DLS is re-skins of paintshop models.
ns