Whole books have been written on the questions you asked, and if there was an easy answer chess would not be very interesting, but here is my two cents:
1) Before making your move, ask yourself the following questions:
A) Does my move allow mate in one.
B) Are any of my pieces or pawns unprotected.
C) Does my opponent have an immediate threat.
D) Is my intended move safe, that is, not immediately losing a piece or pawn for free.
2) Many strong players take a Chocolate Bar and/or Banana to games, the extra sugar and potassium helps keep you alert during long games. It also helps some players to get up from the board from time time to time and take a walk around the playing hall to clear the mental cobwebs.
3) You can't convert an advantage from an equal position, the position is equal after all. I think you meant to ask how to outplay people from equal positions and to do that you have to show a better understanding of the position and chess strategy in general and that takes years of training. A few tips to help you on your way:
a) Don't panic and go crazy trying to force something, such attempts usually backfire badly.
b) If you are stuck for a constructive plan, try improving the placement of your worse placed piece.
c) Think prophylactically, that is, if you do not see an obvious way to improve your position, try to find a move that stops your opponent from improving theirs.
D) In quiet positions think generally on your opponents move and concretely on yours, by that I mean when it's your opponent to move think big picture like where are the weaknesses in his position, which pieces you would like to trade etc, when its your turn you have to be more sepecific by choosing candidate moves and calculating them.
E) Last but not least, patience and tenacity are very useful qualities in handling equal positions well, just ask Magnus Carlsen.
Hope that helps you improve your results.