The Juniper is one of the most recognized bonsai specimens and is frequently the first bonsai purchase many newcomers make. Junipers do make very fine bonsai. Although they are not 'difficult' bonsai to manage, they are frequently 'mistreated' and therefore do not last long. Well, we hate to break the news to you, but, if you have a brown juniper, it is probably dead. The most common mistakes made with junipers are over watering AND lack of sufficient sunlight.
Junipers are outdoor trees.
If you have no way to grow them outside, find some nice person who knows about bonsai and can give them a good home. 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.