Material is most often counted with the standard point values: pawn 1, knight and bishop 3, rook 5, and queen 9. These numbers are a guide rather than a law — activity, king safety, and pawn structure can outweigh a small material deficit, which is the whole idea behind a gambit or a sacrifice.
For improving players, the single most important habit is to avoid losing material for nothing by checking what each move hangs. When you are ahead in material with no compensation for the opponent, the right plan is usually to trade pieces (not pawns) and steer toward a winning endgame where the extra material decides.