Dis, I can't offer anything as constructive as Swede and a couple of other GetBiggers on this subject but I'd love to be a PS expert, but as Swede says, "It takes more than half a brain!" and mine is somewhat less than that even though I read lots of PS material.
Once you become a PS expert, you might want to send your stuff to the major movie studios because they provide an opportunity for PS experts to make their big name stars look better than they actually look in real life... even to the point of placing a star's head on a more impressive unknown's body which I understand is relatively easy to accomplish.
I also suggest that once you are really good, you might want to research forthcoming films on IMDB and produce some possible movie posters for the studios' consideration. Your work could possibly impress them more than their in-house graphics artists.
I've seen this work when Joe Weider received a suggested ad and cover shot from one of the on-line BB fans.
And all of this stuff can be done at home with no need to cross the US border.