Make sure the presser foot is down. Seems silly, but it's not always obvious and is easy to forget about. Also make sure the bobbin tension isn't too lose. Often it just pops out of the bobbin hole but still gets fed through anyway with no tension, sewing a mess in the process. As you keep practicing, you'll be able to hear when your machine has one of these hiccups and be better able to deal with it.
Don't give up!