Well, what fertilizer you use depends on what your bonsai is. I got into this by purchasing a bonsai "seed kit". (Do not do this! I was lucky enough to get mine from a reputable dealer in Disney's Epcot. If you get one from just a mall store, it probably won't sprout like mine did.) It contained a bunch of azalea seeds. I still don't know what kind of azalea, but I am lucky in that my local Meijer garden center carries an azalea slow-release soil acidifier and fertilizer. I don't use the full dosage because my plants are so small, but it works. See if you can find something for your plant before testing random products. 777 is the NPK content, Nitrogen, Phosphorus, and Potassium. The concentration changes depending on the season. For instance, my spring mix has a high N and low PK. Also, you will need to change the soil every couple of years. There are guides for repotting and root pruning, but the general idea is to give your tree room to grow for another few years, as well as add a little extra nutrients and allow the water to pass through a little more freely. I personally purchased the Reader's Digest "Bonsai Secrets" to teach me all of this. You don't have to get that exact book, but it's a good idea to have a guide of some kind. If you find a great internet site, that's fine too, as long as it has the information you need.