As everyone else has said, you will indeed want to start off with some lessons.
But to answer your question, when you've decided that English is for you, then an All Purpose will probably be what you're looking for to ride comfortably on the trails. Look for a used quality saddle rather than a cheap new one. Unless you have a bunch of cash to spend, then get something new and good. Try lots of saddles before you buy. Even in the All Purpose category, you'll find flat seats and deep seats, knee pads and knee blocks, or just plain flaps, wide twists, narrow twists, rock-hard and padded, etc. Find what you and your butt like before buying anything. Also note that an English saddle has to fit your horse much more precisely than a Western saddle does. Take a look under Tack and Equipment. There's a sticky called: Does your saddle really fit. Also lots of posts in the tack and equipment threads about saddle fit and sizing.
You'll want a thin pad. In English, the pad is pretty much there to keep the saddle clean, not to make the saddle fit.
You'll need some kind of snaffle bit. In English, you ride with constant, light pressure on the mouth, pretty much just feeling it, and use direct reining. No loose reins, so you'll be wanting to get away from any shanked bit, at least to start out with.
You'll also need 1-piece reins, no split-reins.
And have fun!