With openings, I've found that Sicilian openings are extremely popular (even more so than the Ruy Lopez), so you would definitely want to study up on this one as well in my opinion.
When I was just a kid, I was taught a couple of beginner strategies in the opening that I still use today.
1) Control the centre, so you have more space for all of your pieces. One of the worst things is being in a middle game with all your pieces cramped up so they can't move.
2) Develop your minor pieces (bishops, knights) and try not to move them more than once (unless you have a good reason to) so you don't waste time. Wasting time in the opening is also really bad, because then you will be behind the whole game!
3) Don't develop your rooks and queen like you do with your minor pieces as if they get attacked, they'll have to move and will waste time. Just let them sit on the back 2 ranks in the middle of the board somewhere (after castling) until the middle game.
4) Castling. This is an absolute must (even though it is only 1 move). It gets your King off to the side of the board where it is usually in less danger than in the middle and gets a rook more into the middle of the board.
Being able to plan ahead is critical in chess. If you can't think of what you will do in the future or what your opponent will do, then you will really have no chance. I'd say that 3 full moves ahead for tactics would suffice the majority of the time. Strategy is also important, the ultimate aim is to checkmate of course, but you need to have strategies in the game as well. They might be as simple as getting 2 rooks on the same file because the opponents back rank is weak (which may only take a few moves), or it might be as complex as getting all your pieces into a specific position (which may take 20-30 moves). Like SMX said, Strategy is probably the least important of the 4 phases, but you still need to know about it.
Speaking about Tactics, I found a book probably about half a dozen years ago that really helped me with my tactics. It's called "Winning Chess Tactics" by Yasser Seirawan and contains beginner tactics like forks and pins (that SMX has stated already) and works all the way up to more advanced tactics. You definitely need to know some basic theory for it to be useful, so once you feel you have a good grasp of beginner theory you can have a look for this book.
Another thing I would look at is common checkmating patterns, because once you start playing, you'll notice that a lot of checkmates are repeated over and over. Knowing these checkmating patterns means that you can plan ahead for what sort of checkmate you want. If your opponents back rank is weak, try to aim for a back rank checkmate with a rook or queen. If some of the pawns in front of the opponents king (after castling) are missing, try to use bishops, knights and your queen to get into the holes to Kill the King (sorry, just had to
)!
But of course, the best way to learn how to play competently is to play against better players and learn from them!