To bind off on a purl row, just purl 2 sts, and slip the 1st over the second. (If you were to continue binding off, you'd just *p1, slip previous stitch over this stitch, *and repeat.)
To join in yarn for the left half....
You see, you've used the yarn to make the right half, and there's no yarn to work with dangling from the left half. So, you just start knitting with a new piece of yarn, preferably from the under-arm side, where there will later be a seam, for easy darning in of the yarn's end. After the armhole is sewn in, just thread that yarn end onto a tapestry needle, and run it through the sewing stitches for a couple of inches. Or, if you want to get it over with sooner, you can just weave it into the adjacent stitches, following the path of a single strand of yarn (duplicating the stitches), for about three stitches. Cut it, leaving 1/4" - 1/2" tail of yarn on the inside, so it won't pop out on the outside.
Way to go on starting a more challenging project! Let us know how it goes, and if you have more questions! If you have a digital camera, I'd love to see your progress!