If it is a juniper then it will do best outdoors in a sunny location, these are not indoor bonsai.
It sounds like your over watering it. Here is a link on how to properly water your tree,
Over watering will rot your roots and kill your tree, let the soil dry out almost completely dry before watering it again. I use toothpicks on my smaller bonsai and chop sticks on my large bonsai, it is fast and easy.( read the link above to learn more about this )
There is allot of information about Junipers here on this forum, just use the search and type in " juniper "
Good luck and hope your bonsai gets better