Knowing which pieces to sacrifice and when to sacrifice them is probably a matter of opinion as much as anything. There are of course exceptions where a sacrifice is just plain silly. When and why to sacrifice a bishop is often a matter of opinion since some players see a bishop as being highly valuable while others see it as having little value.
One way to gauge that value is to look at what is called the unopposed bishop. An unopposed bishop is one whose counterpart on the opponent's side has been taken. In other words, if you have your dark square bishop and your opponent does not, then you have an unopposed bishop. The opposite of this is true if you have lost one of your bishops and your opponent still has theirs that is on that square color then your opponent has an unopposed bishop.
The primary way that an unopposed bishop is dangerous is that if you are being attacked by an unopposed bishop you cannot block with your own bishop. If your opponent has an unopposed bishop then your opponent can also use its to start taking down your pawns and your primary line of defense. An unopposed bishop becomes even more effective in the endgame with its ability to threaten the opponent's king without having to guard against the opponents bishop of the same color square.
An unopposed bishop can become quite a valuable asset if you are aware of how to best utilize it. Often though many inexperienced players will sacrifice a bishop in order to keep a knight. This may be an appropriate move in some circumstances. The point is not to surrender your bishop and to allow you opponent to get away with an unopposed bishop.