When you start in the round, you have your stitches going all the way around the circular and the working yarn will be coming from the first stitch on the needle tip in your right hand. So when you work your first stitch on the left hand needle, using the working yarn that is coming from the right hand needle, this is what produces the join in circular knitting.
If using circulars to knit flat, you start off with ALL of your stitches on the needle tip in your left hand (not literally, because of course it is
still joined by the cord), pick up the other needle tip with your right hand and start working the stitches. Your working yarn in this case will start out attached to the first stitch on the needle held in your left hand.
Like brendajos said, once you get to the end of your stitches, you turn the work around and pick up the opposite needle tip and begin your next row.
By the way, there is no such thing as a dumb question!