Dustinac gave you directions for the most basic join. It works just fine and I used it (and only knew it for many, many years). There are some other ways to join. Join just means to connect the cast on stitches into a circle for knitting in the round.
A simple join that I like to use now is this. Cast on one extra stitch to do the join. (When the join is completed you will have the number called for in the CO directions.) After you have checked to make sure there are no twists (the way I do this is to make sure all the cast on edges of the stitches appear on the inside of the circle formed when I bring the ends of the circular together) slip the last stitch you cast on (it will be on the right hand tip of the needle) over to the tip of the left hand needle. Now work that slipped stitch together with what was the first stitch on the left hand needle. You can knit the 2tog or purl them together, whatever you are suppose to do. This makes a neat join.