By "I don't know how to start" do you mean you don't know how to crochet and need to learn? Or do you mean you don't know how to attach the yarn to the knit blanket to begin crocheting or something else? To join the yarn you just make a slip knot on the crochet hook and do a slip stitch into any stitch you'd like to start in (it doesn't matter, although I would not start smack dab on a corner if it were me), chain one and begin single crocheting around.
If you are looking for stitches you might use beyond basic sc or dc there are sites that have crochet stitches. Try doing a search and see what you find. If you need to learn to crochet do a search for sites that teach that. There are probably sites similar to this that have help and videos. I know youtube has some videos.