I would suggest starting with Tactics and Endgames. You need to spend very little time on Openings. Why? Well, your opponent is very likely to deviate from theory pretty soon so a solid tactics skills is needed to really benefit from the odd stuff you will experience.
Solving Tactics problems is "learning by doing" and Openings studies tend to be (as GM Davis accurately put it) "learning by nodding". It is very easy to fall into the trap to try to memorize stuff when studying openings. Very few books on openings help develope an understanding of the finer points of the openings.
I've played chess for 20-25 years, since my dad taught me when I was a kid, but never with much frequency and always casual games against other unstudied players, so I consider myself very much a beginner. I've recently decided that I'd really like to put some time into studying and practicing so that eventually I could claim to actually be good at this game. Unfortunately, I'm kind of at a loss as to where to start. It seems intuitive that the first thing I should focus on studying is openings, but beyond that I really don't know. Any advice as to how to proceed would be greatly appreciated. Also, I'm looking for a good book on openings, (something with a good general introduction for a novice like me) and if anyone has any recommendations that would be great too.