To decide on a trade, ask: who benefits? If you are ahead in material, trade pieces, not pawns: the endgame gets easier. If you are behind, keep pieces on the board and look for complications.
If you are attacking, trade your opponent's defending pieces; if you are defending, trade their attacking pieces. Trading your bad bishop for your opponent's good piece is also a good deal.
Before trading, picture the position after the trade: what will the pawn structure be, and which pieces will remain?