I'm going to show you how to join to knit in the round.
I'm using circular needles and I have cast on almost enough stitches to go all the way around from tip to tip. You want to make sure that you don't have to stretch your knitting too much when you go around, so I'm going to cast on just a few more. I'm using the long tail cast on. Let's cast on just a few more so that my knitting can comfortably stretch from end to end. Great.
Once you decide that you've got enough stitches to go around, or however many stitches your pattern calls for, you're going to do a joined knit in the round. What this means basically is you need to make sure you have no twists in the knitting. I'm going to look at my cast on edge and make sure that it goes around, it never ever twists. A twist would be something like this, and if you joined the knit in the round now and knit around, you'd basically be knitting a mobius strip and we will never not have a twist in your knitting, which can be problematic.
So, once you've established there are no twists in your knitting, you're going to join in this way. With the yarn coming from the stitch in your right hand, in your right hand needle, you're going to insert the needle into the first stitch that you cast on, which is maybe your slip knot, depending on what kind of cast on you used, and you're going to use the tail from the last stitch to knit a stitch into your first stitch. I'll go a few beyond it here so you can see that now, here's the tail from the end of my cast on, and here's the first stitch that I made.
Now, they're joined, so I can safely now knit round and around and around without stopping and not have to worry about twist. However, there is a small danger when your project is this new and this small, that you could put it down in a way that actually twists the knitting around itself. So, if you're going to put your project down when it's still short, make sure when you come back and check, it never hurts to check to make sure there are no twists.
So, this is a successful join to knit in the round.