It's a very relaxed way of riding. It shouldn't be too hard to transition, but I've ridden both english and western for quite some time, so I may not be the best one to talk.
A western saddle should fit you very well, and it should be very comfortable. At first it might feel as if there's a lot between you and the horse, depending on what type of saddle you're riding with.
I find that my horse tends to prefer western, because he can just kind of do everything slowly (he's the slowest of pokes) and he doesn't have to have a higher head carriage like in english.
Your legs should hang so that if I were to draw a line from your ear to your hip to your heel, it would be perfectly straight. Heels down, toes pointed slightly outward. Your upper body should be straight up but relaxed and your lower body will go with the horses movement.
It might take a while to figure out a jog/trot, but you'll get it!
After you ride - if you have questions, come back and ask!