When joining in the round, it’s very easy to get a little gap in between the first and last stitches. You can stitch this up when you weave in your ends, or you can try this technique!
Slip your first and last stitches onto a spare needle, then twist that needle around. Now, the first stitch will be on the left, and the last stitch on the right. Slip the left stitch onto the left needle, and the right stitch onto the right needle. This twist actually crosses that first and last stitch to close your gap. Now you can start knitting without having to worry about the gap again!