If you want to try a juniper anyway, make sure it gets plenty of sun, don't over water it, and make sure it gets plenty of air. If you have an ailing juniper, and if you have managed to catch it early enough, you may want to remove the tree from its pot and bare-root it. Then repot it in pure sand. This is an extreme measure. Many people make the mistake of assuming that since bonsai is in a pot, it must be kept indoors.
Some people get into bonsai just because they want to have a tree inside.
Most of the trees you will see in the books and magazines, however, are actually kept outdoors most, if not all, of the year. Also, unless you have an attached greenhouse, the sun will be getting blocked off for at least part of the day due to walls, the roof, etc. I would suggest getting some extra lights for your trees. Total cost was about 25 dollars. I leave the light on from 14 to 16 hours per day, and keep my plants as close as possible to the tubes.