Two ways to go about what you are wanting to do (I am a degreed Graphic Designer, and have experience doing paint schemes, as well as vehicle wraps).
Paint work: You can use PS to show the paint shop what you want the finished product to look like. The shop will go by your renderings, mix the paint to match, tape off by hand, etc, and come up with the finished product. I HIGHLY recommend taking both truck and trailer to the shop, and LET THE SHOP do the measurements on where the graphics crossover to the trailer.
Vehicle wrap: Again, you can work up the graphics to your hearts content. But what you work up will be the final design the shop will go with. If they don't build the graphics from scratch themselves, with their own measurements, they probably won't guarantee crossovers and such. Also, keep in mind that the wrap graphics are printed process color, and may or may not match the actual mixed paint on your trailer. There is a TON of stuff you need to know to prep the file correctly. I highly recommend letting the vinyl place build the graphic files themselves.
As agesilaus mentioned above, PS is a VERY complicated program for someone to learn on their own.
Cliff notes: if using real paint, nothing wrong with whipping up the general idea of how you want the graphics to look in PS.
If using vinyl wrap, I highly suggest you let the experts build the design for you. You can give them a general feel of what you are looking for. They will need access to both the truck and trailer to get it done correctly.