Jalopy is right. You should post a photo. Also, drainage is VERY important. Right up there with air and light and water. Also consider where you bought it from. Roadside dealers are not usually considered as 'reputable vendors'. But that's okay. You may well have rescued this tree from horticultural genocide. My first suggestion is repot as soon as possible. Either take it out and drill some good size holes in the pot it came in or find a slightly larger, made-for-bonsai, pot. Or even put it into a good size nursery pot for a year or so. Considering where you acquired it from there's a very good chance that the soil that came with it is totally wrong for it. It's probably standard potting soil. Retains way too much moisture. Especially for a juniper which likes a somewhat drier soil (I think). For the soil you can find a commercial bonsai mix or, in a pinch, use a soil formulated for cactus and succulents. Ask around on this forum and you can probably find some much better soil suggestions than that (get 10 bonsai people together in a room and you can get a lively debate going about soil
) After repotting take it out of the sun for a week or so and let it rest and recover. Gradually start moving it back to a sunnier position. You probably won't have to worry about root pruning, especially if you move it into a larger pot. Should be okay. Hopefully others in this forum will give you some more advice and I certainly defer to the vastly greater knowledge and expertise that can be found here. Just giving what I hope are good suggestions. Especially considering that I've killed three junipers (well, I killed two and a raccoon dug up and dragged off the other lol). Hope this helps some. Rescue trees can make great bonsais!