I would disagree about learning openings first, but suggest what others on chess.com forums say, and that is to learn tactics. Learn the basic mates, King + 2 Queens/2 Rooks/1 Queen/1 Rook/2 Bishops vs King, and find the most efficient way from different positions (skip King + Bishop + Knight vs King, it is not easy especially for beginners). Avoid stalemate! Sometimes it is very easy to fall into one when you don't think about it with something like King + Queen vs King. Chessvideos.tv provides an endgame simulation for you to practice them, and videos to show you how to mate with them.
As for tactics, solve a lot of puzzles. Chess.com has tactics trainer, chessvideos.tv has puzzle of the day, chesstr.com, chesstempo.com, shredderchess.com has a total of 9 puzzles (Today, Yesterday, Day Before with Easy, Medium, Hard level; obviously if you visit everyday then you work with 3). You can also check out chessgames.com, on the home page they have a puzzle to solve, but only solve the easy ones (1/2 star max), as they can be really challenging going any higher.
Avoid hanging pieces, that is you left a piece on the board that can be captured and isn't being protected by one of your pieces. Look for when your opponent does this.
Tactics you need to know w/ Wikipedia definitions
- Pin: A pin is a move which forces one of the opponent's pieces to stay put because moving it would expose a more valuable piece behind it.
The bishop is pinning the knight, meaning it can't move without exposing the king. When you do this with the king the piece you are pinning can't move at all, but like the definition said, it can be any piece that is usually greater than the piece that is being pinned.
Skewer: A skewer is a move which attacks two pieces in a line, similar to a pin, except that the enemy piece of greater value is in front of the piece of lesser value.
Just reverse the pieces of the pin.
Pawn moves: Moving the pawn to move a more valuable piece.
Fork: A fork is a move that uses one piece to attack two or more of the opponent's pieces simultaneously, hoping to achieve material advantage, since the opponent can only counter one of the two threats.
All of whites pieces forking two pieces. The knight fork is something you need to pay attention, as even very skilled players fall for it, and is quite effective.
Discovered Attack: A discovered attack is a move which unmasks an attack by another piece.
You can also create double checks which makes their king have to move.
Sacrifice: Sacrifice is a move giving up a piece or pawn in the hopes of gaining tactical or positional compensation in other forms. This includes eliminating a piece in lower value to the sacrificed piece.
This may include "Removing the defender" which may take away added protection from a piece, expose the king more, or something that will give you an advantage.