You can't ride in an English saddle without a girth! But as you stand in the aisle of your local tack shop staring at the various girth options, you might not be sure which one to buy. Each brand tends to make a few different girths, and the options are basically endless. However, girths can mostly be broken down into the following types. If you decide which type is best suited to your needs, then you just have to pick the one that you find most attractive and in-line with your budget.
Simple Leather Girths
This is the most versatile type of English girth. Simple leather girths are about 4 inches wide, and they may be contoured a bit to come back away from the legs. They have buckles on both sides. Some have elastic on one side or both sides to make them easier to tighten. Simple leather girths are great for schooling rides since you can easily wipe them down to clean, but they are also classy enough to show in.
Fleece girths are usually made from a synthetic nylon material backed with a thick layer of fleece. These are a great choice for horses who are more sensitive in the girth area and who tend to develop girth galls. They can often be put in the washing machine to clean. Fleece girths are not necessarily appropriate for the most formal of shows, but they're certainly fine for schooling shows and open shows. Some people do not like how shavings and horse hair stick to the fleece, though.
Shoulder Relief Girths
Do you have a horse with big shoulders or a lot of shoulder action? A shoulder relief girth might be right for you. This style of girth is cut back with a bit of a "u" shape so that it does put pull the saddle down so tightly onto the shoulders. Shoulder relief girths are usually made from leather, and they're suitable for showing and schooling.
If you specialize in jumping, then you may want a jumping girth. These girths have a broad pad that goes between the legs and onto the belly to protect the horse as the horse folds their legs up underneath them. They're really only necessary for horses who are shod, and for those who tend towards the higher fences. If you're only jumping 2'6" or so, you may not need a jumping girth, although there is no need for one.
Hopefully, you now have a better idea of which type of English tack may work best for you. It's a matter of preference, mostly.