Welcome to the site.
Here's my Juniper...got that right yes?
Bought it from home depot a week ago, got rid of the glued on rocks and after four days inside, got it ouside per forum readings.
Removing the rocks was the right move and although you are correct that Junipers should be kept outside I am not sure I would have put it outside just yet. My fear is that it is not acclimated to the cold temperatures and that the sudden move might cause trouble. From this spring on it will never be brought inside again, (some winter protection can be required though)
I've noticed the soil it's in doesn't seem to be too grainy,
seems like regular soil to my untrained eye. retains alot of water too.
I agree, unfortunately mass produces bonsai are seldom potted in decent soil. By the looks of this and the time of year I don't think you will need to water again anytime soon, especially if you keep it in a cool location as you have done. As long as the soil has some moisture in it you are good.
I also am worried I over watered in the second day I got it. I put it in the sink with the water level up 3/4 of the way, but forgot about it and it was in there for like 30 40 mins
Don't worry about it. you do not over-water a plant on one occasion, that is an ongoing process. Make sure not to water again until it really needs it, not because you think it must be time. Check frequently and only water as necessary.
Can anyone offer some guidance to a newbie?
As I noted above being in a very cold location now, coming from a fairly warm greenhouse, is problematic. Do you have any options other than bringing it inside? Plan on re-potting it in late spring or early summer. Read the sticky threads at the top of the forum and begin researching proper bonsai mediums. We'll be here to help as you discover more questions.