There is a point system where the queen = 9 points, rooks = 5, knights and bishops = 3 and pawns =1. It's a good way to evaluate what to trade off during a game but it can also be misleading. I rather believe that the pieces change in point value as the game progresses due to the player's ability to use them plus their position on the board. For instance, you might be extremely good with knights and not so good at mobilizing your bishops. So, for you, you might want to rate your knights at 4 instead of 3. Also, a 1 point pawn on the 7th rank is just about to be queened. Certainly that pawn is worth much more then than it was when it started out as a 1 pointer. Likewise, you will need to adjust the point value system to match your opponents abilities with certain pieces. They may be exceptionally good with bishops, so trading a rook for a bishop might be more disabling for him than a rook for rook exchange.
Keep in mind that when one bishop is captured, the remaining one is always on the same color throughout the rest of the game which can be extremely limiting. Where as knights alternate colors on every move which in my mind is a little more valuable. So, a lone knight can still capture on any color, but a lone bishop cannot.
It's probably best to start with the standard point system mentioned, and then after learning your strengths and weaknesses, customize the point system to suit your own game play. I hope this helps. Good luck and castle early!