Don't use a brush on the larger surfaces, it usually adds to much epoxy, if you do use a brush then you'll need to go over it afterwards with a credit card or similar to scrape off excess epoxy. I usually do it like this; cut your fiberglass cloth to size and place on the surface to be covered, then I mix up a batch of epoxy and just pour a small amount onto the surface and spread it out with a credit card or similar tool, I only use a brush around curved sections and around the edges, the goal is just enough epoxy to fill the weave of the cloth, it should not have a shiny surface after it has cured. Only pour on a small amount so it wont take you much time to spread it out, if it sits too long in a pool on the balsa, it will start to soak into the wood too much. Then trim around the edges and do the other side, then when that is also cured lightly sand with 240-320 grit paper, clean and apply a second coat of epoxy, this second batch I like to mix 50/50 with microballoons as this helps a lot with preventing pinholes and also is easier to sand smooth.
I use 25gram cloth, and please get some quality material as it will save you a lot of finish work. I get mine from www.r-g.de
, their 25gram cloth is really nice. It is tighlty woven with a smooth finish, some of the cheaper stuff usually has a more uneven texture and surface which means more sanding is required for best results. The best you can do if this your first time glassing is to start with something easy as the tail surfaces and use those to practice on before doing the larger more complicated stuff like wings and fuselage.