These answers to these questions depend upon the exact position, but i can give you a couple of general ideas to look for. One thing to consider is that there are both offensive and defensive exchange sacrifices....You might sac your rook in order to bring about a winning position, to increase the scope of the rest of your pieces, shatter a king's protective position (all offensive sacrifices), or you might have a need to remove a dangerously posted knight, or at least one that is interfering with your plans (a defensive sacrifice). These defensive moments are true judgment calls, often the only way to save a game, and your knowledge of whether the resulting endgame is a draw or loss is just as important as the evaluation of the middlegame situation. The fact that there are so many factors to consider is what keeps chess interesting, and what makes it a great fight.

A knight in the center of the board can control a maximum of 8 squares, while a bishop in the center can control 13. So, as a rule, bishops are a little stronger, especially when the two bishops are working together. The fact that knights can cause trouble and are a little more "tricky" tactically is probably what causes some people to "like" them more. Only in a very closed position, or in an endgame where the pawns are on the same color squares as the bishop (and thus restrict their movement) are the knights generally preferred. A knight in the center of the board can control a maximum of 8 squares, while a bishop in the center can control 13.

I would highly recommend gambit play in the beginning steps of your development as a chess-player. It is a terrific way to learn the balance of the various elements of chess play: time, material, pawn structure, and space. It is also helpful to be on the defending side of a gambit, so if you have a friend or chess partner, trade off every other game in playing gambits. It is a fun way to get better at chess.