Ingrid is right, but I'd like to add that you should be careful to make sure your knitting isn't twisted before you begin the first stitch that joins. The way I do this is to look the stitches over carefully and be sure that the cast on edge of all the stitches is on the inside edge of the circle of the circular needle as I am holding it, ready to join.
The most basic way to "join" is just to start knitting, but there are other ideas out there about ways to join in the round. If you'd like to explore some of the other ideas you might find them with a search like "circular join". Good luck.