The bonsai tree routine is taking it outside when it’s sunny. When I leave it outside, it sits there for about 5-6 hours, mostly from afternoon to sunset. When it’s inside, I still try to set it in sunlight, like by a window. The bonsai is watered every other day at first but when it started brownish, I thought I was overwatering, so I’ve been watering it every 3 days. I let it drain out, wait 15 minutes, then  water it again when I’m watering it every 3 days. It started turning brown (some of the undersides of the leaves) a week after I purchased the bonsai. It’s 9 years old and is a juniper bonsai tree. 

I haven’t bought fertilizer for the bonsai yet because I’m trying to figure out what fertilizer would work well for it.

How do I stop it from turning brown? I got it several weeks ago. Does anyone have tips in general for bonsai tree caring or if I'm doing anything wrong?

Why do you keep moving it? Put it outside and leave it there. You are causing the problem if it is still alive by taking it inside. Junipers are hardy and do not need protection, and will DIE inside a house very quickly.