Assuming you are going to add another skein of the same yarn: when you get to the end of a row & it's obvious to you from looking at the yarn that you won't have enough to crochet another whole row, it's time to add the new yarn. Easiest way: When you're working your single crochet, there's a moment when you have 2 loops of yarn on the hook. Normally, you hook the working yarn (from the skein) and pull through. So, at this piont, grab some of the "new" yarn instead of yarn from the old skein, & complete the stitch with that.
This is the "correct" way to attach new yarn, & you might want to practice it, because it'll play into changing colors as well.
HOWEVER, as long as you end at the end of a row, there's a way you can fudge it. You can just finish off your work at the last stitch & attach the new yarn by slip stitching it in that end stitch & then making your turning chain with the new yarn.
Either one will work. I'd suggest you practice the "correct" way, though, for two reasons. One, like I said it comes in to play when you're changing colors. Also, though, sometimes it's easy to space out & run out of yarn midway through a row. If you know how to do the "correct" way, it's no big deal to just attach the new yarn there & go on your merry little way. (Even though it's recommended to attach new yarn at the edge, it really doesn't make a visible difference to do it mid-row if you do it the proper way & weave in your ends.)
Second topic here: what to do with those ends. Personally, I tie the yarn ends together, just once. Then you can either hold them carefully at the back of your work and crochet over them, or leave them until the end & weave them in like you should. I have heard that a little dot of fabric glue on the very end of the yarn you weave in will prevent it from popping out to the right side of the work; however I haven't personally tested this.
