I would say your best bet would be to wait, as well. while you're taking the class, however, make sure the professor gets to know you. If you do well on your exams, your performance on them should be enough for the prof to write a letter for you, but you can also improve that recommendation by participating in class, visiting your prof once or twice during office hours, etc. If the class is large and there's a chance that prof might not be able to put your name to a face at the end of the semester, it's important that they know who you are. Just don't be a gunner - the obnoxious one who speaks just to hear their own voice.