Strings are marketed under general categories when we're talking about thickness, or "gauge." There are variations of these but most people choose one of the following: Extra light; light; light/medium; or medium.
Although you may find some very slight variations from one manufacturer to the next, here are how those categories generally break down in diameter (low E to high E, all listed in inches):
Extra light: .050, .040, .030, .022, .015, .011
Light: .054, .042, .032, .025, .016, .012
Light/Med.: .055, .0435, .0335, .0255, .0165, .0125
Medium: .056. .045, .035, .026, .017, .013
I should add that some strings from certain manufacturers are formulated to have a softer or firmer "feel," which is related to the amount of tension that is needed to bring the string up to pitch. Keep in mind that a softer feeling string may be easier to press down but buzzes are more likely to develop as you play due to the lower tension of this type of string.
So which gauge should you use? That is difficult to answer but you should consider your overall playing style (if you strum aggressively you probably should use mediums) and the size of your guitar (smaller bodied guitars are best used with light strings, although that is subjective). It should be noted here that although almost all guitar makers will say that their guitars will hold up just fine with long term use of medium gauge strings I have seen more than a couple very nice acoustics that needed neck resets way before their time and although I can't prove it, I think that the use of higher tension/thicker mediums contributed to this. I also feel that living in a climate with seasonal variations in temperature and humidity exasperates this.
Mediums do bring out most of the sound your guitar is capable of delivering, no doubt about it. This is why they are preferred by bluegrass guitarists who's job it is to provide loud, driving rhythm. Finger style players who want clarity and also need to be able to run through complex musical passages without being impeded by a string that needs a forceful attack usually prefer light gauge. I happen to fall into that category, even when I'm playing a large bodied guitar like a Taylor 810 or a Martin dreadnaught. I'm willing to get a bit less volume in exchange for ease of playing. You may want to try a set of the light/mediums to see if these "compromise" strings accomplish what you're looking for in both strumming and finger style playing.
A couple final thoughts about strings. We're fortunate, compared to, say, violin players in that our strings are quite inexpensive compared to theirs. A very fine fiddler I used to play with used to order her strings from a company in France (!) and she paid about 10 times more than what I paid for my strings. So the point is: change your strings fairly often to keep your prized guitar sounding as good as it can. If you have dry hands and play at least a few times a week, think about changing your strings every 2 - 3 months; if your hands get sweaty when you play you'll need to change your strings much more frequently.
I'm not going to get into the mechanics of string changes at this time but I know there are plenty of You Tube videos that explain the process. All it takes is a few minutes of your time and less than $10 out of your pocket, in most cases. I have an extensive variety of strings available both on this site (Guitar Accessories page) and over in my online store:
Don't be afraid to experiment with different gauges and brands of strings. You're sure to find one you like. And just remember - putting new strings on your guitar not only makes it sound at its best but knowing your guitar is sounding good will inspire you to pick it up more often!
Peace & good music,