I tried gliding a few years ago - I had a trial session and then went a few times to three different clubs. I never managed to devote sufficient time to it to go solo, maybe I will return to it when I am older/retired, hopefully.
OP - definitely have a trial session at a local club. If you like it, consider the following if you have a choice of local-ish clubs. Don't necessarily sign up with the nearest one. If you need to drive (or cycle!) an extra hour to get to a better one, it can be worth it.
1. Do they have a winch? You really need a winch as a beginner, as it's the best and cheapest way to practice circuits and landing. Aerotow is good later on, I believe.
2. What's their launch rate? If they share the airfield with something else (e.g. parachuting), their launch rate will be lower and you'll spend more time sitting around. Also, if they only have a small number of aircraft, the launch rate and your learning progress will be lower.
3. What are the people like and the facilities like? This is hard to gauge from a trial, but you can get some idea. If they are mostly unfriendly and/or cliqueish, the time you inevitably spend sitting around will be extremely boring. If they are friendly and involving, you will be more likely to partake in and enjoy the activites on the airfield apart from the actual flying (e.g. getting aircraft ready, helping in various ways, classroom sessions...)
4. Are they well organised? Do you get the feeling that if you wandered off for a few minutes, your turn would be missed? Or do they have a blackboard system and good marshalling. That sort of thing.
5. What are the costs per launch and the cost while in the air? How much is membership? Gliding costs are pretty low, but that's relative
6. Is the instruction free? (it normally is, I believe).
7. Obviously check out their website, if they have one.
I'm speaking from limited experience, so someone else may add more things to look for.
Depending on where you are and your disposable income, there are other options than a voluntarily run club, such as
http://www.lashamgliding.com/. I've never been there, by the way.