Since this is your first western saddle, go for a all purpose one.
Horse: There are several different tree sizes in each saddle. There is semi quarter horse, quarter horse, & full quarter horse bars. Semi quarter horse has a gullet of 6", quarter horse is about 6 1/4", & full quarter horse is about 6 3/4"-7". If you have a very wide shouldered horse, go for the full. If your horse is a medium or medium wide in english saddles, he would be a full quarter horse in western saddles.
Rider: You should fit pretty snug in the seat of a western saddle. You want to be sitting deep with no gap behind you. There should be 2-4 fingers width between you & the pommel of the saddle.
Bridles & what not:
You can ride in snaffle bits in western. If your horse already works well in a snaffle bit you can use that. Or you can go on horse.com & look at the bits in the western section & find which one suits your horse. It all depends on what discipline you are planning on riding in western. If you choose to do barrel racing, you'll need a mechanical hackamore or a gag bit. If you are doing western pleasure you'll need a fancy shanked bit. There are a lot you can choose from. The reins can be split or loop. Depends on what you are comfortable working with and which western riding sport you are doing. The headstall can either be one eared of have a brow band. There will be no cavesson on it.