I am by no means a Chess Grandmaster, but I play enough Chess online to notice the exact same mistakes being made by beginners over and over again.
The opening phase of a chess game dictates the flow of the entire match. If you mess up the first five moves, you will spend the rest of the game desperately defending. Here are the three unbreakable rules I follow every time I sit down at the board.
Rule 1: Control the Center
The four squares in the exact center of the board (d4, e4, d5, e5) are the high ground. Whichever player controls these squares controls the game. Your first move should almost always be moving your King's or Queen's pawn two spaces forward to claim this territory.
Rule 2: Develop Knights and Bishops Early
Your powerful pieces are useless if they are stuck behind a wall of pawns. Focus on getting your Knights and Bishops out into active squares before you even think about moving your Queen. A common beginner mistake is bringing the Queen out immediately; she usually just gets chased around the board while the opponent develops their pieces for free.
Rule 3: Castle Early
Your King is vulnerable in the center of the board. Castling tucks your King away into a safe corner behind a wall of pawns and simultaneously brings your Rook into the action. Try to castle within the first 10 moves.
Stick to these three rules, and you will instantly see an improvement in your win rate.