Figure out what color quilts you usually make and then buy your quilting thread. I like Glide thread. It's inexpensive and my old long arm likes it. I always buy the large cones and usually get two at a time. That way I don't have to unthread from the long arm if I have to make another bobbin. I have a stand alone bobbin winder. The colors I use most often are white, light beige, wine and black.
The idea about the old sheets is good. If you go ahead and put batting in it, it can be a utility quilt you use for picnics, or anything else you need. There are Cheater quilt tops that look like they are pieced but the pattern is just printed on the fabric. They are great for practice. I have seen them quilted so well that you could not tell they weren't pieced. I have rulers and other shapes and find I don't use them. I haven't built up those skills yet. I use the large corsage flower pins from Wal Mart to anchor my quilt to the leaders and to pin down the side as I am quilting. You cannot roll with them in the sides, they must be removed. Yep...tried it!
Have fun!