One of the most fundamentals features in Photoshop is the Pen Tool. It's a bit less intuitive than some other features, however, and it often presents some difficulty for first-time users. This video will get you up and running with it in 30 quick minutes.
I'm going to be honest: I used to hate the Pen Tool. It's not Adobe's fault at all; I was just terrible at it. Learning how to use it was like trying to speak French in Russian for me. Nonetheless, my inexplicable weak spot does not make the Pen Tool any less valuable; in fact, it's far and away the best tool for precise, complex selections, making it invaluable, particularly if you're working on composites. Having a helpful tutorial like this one from PiXimperfect would have made a big difference in my learning progression, however. In it, you'll start with the basic elements of the Pen Tool and how it functions, gradually building up to more complex paths and selections and applications. It's a bit tricky at first, but once you learn it, it'll likely become an indispensable part of your photo-editing arsenal. As Dinda mentions, be sure to spend a little time playing the Bézier Game for some guided practice.