When should I trade a piece?

I'm always seeing chess games where high level players trade pieces but I can never understand why, even in my own games I don't know exactly if or when I should trade a piece, does anybody have advice?

As a good first estimate, use the point system. Each piece has a point rating and your goal is to remove more points from your opponents set than they remove from yours. Beyond that, extrapolate the same system forward by working out, if after the trade there are further trading or checkmate opportunities after your current trade has been made. You'll work out extra stuff along the way, which is beyond your current rank but I wouldn't trouble yourself with that for now.

#1
"if or when I should trade a piece"
++ Trade an inactive piece of yours for an active piece of his.
If you are ahead, trade pieces, not pawns.
If you are behind, trade pawns, not pieces.
If you defend, trade to relieve the pressure.
If you attack, do not trade to keep pressure.

Also  know this thing that, what is left on the board that matters, if you trade your active piece for opponents bad piece it might be good for you. 

For example. Trading active bishop for a knight in closed positions. Leaving your opponent with a bishop in closed position would give you advantage.

to really understand it you have to visualize the board after the trade and whether its good for you.  Equal trades are rare unless you are very high rated player: most come with a (often hard to see or understand) problem for one of the players. 

Some examples.. 

Recapture with a pawn can be good or bad, but in general doubled pawns are weaker than adjacent ones (not always).  You may want to double your own pawns to let a rook or queen loose, though, open up an attack.  At this point in your study, ... if you do not have a relatively immediate use for opening a pawn hole/doubling pawns, avoid it.   Likewise if you can't see a problem with taking to force your opponent to double up, do so.   

beyond the pawns, a bishop that can't move due to your own pawns is useless for a while, and could be traded for a knight that can get through the blockade, making you stronger and opponent weaker.   A single bishop can't get to the other color squares and if that is a problem, trading it for something may be the answer.  

And so on.  In short, a point system is part of it, but it misses the relative usefulness of the pieces in a given position and the misalignment of pieces after the trade.

The answer to your question is very complex, but here are a few simple ideas to consider.

If you are winning, trading pieces means your opponent will have fewer options for defending and creating counter-threats. 

If your king is under a strong attack, trading off one or more of the attacking pieces may nullify or reduce the attack.

Most of the time, however, the matter isn't this simple. So the things to ask yourself before trading pieces is this: what will the trade accomplish? Some beneficial things that piece trades may accomplish:

• Creating a weakness in your opponent's pawn structure, such doubled pawns or an isolated pawn.
• Remove a very active and well posted piece of your opponent's (e.g., a knight in the center of the board)

Piece evaluation isnt static. Sure you have the general points system, but what the piece is doing matters a whole lot. If your opponents piece is cramping your options and making the game hard for you then getting rid of that piece should be a priority as the activity might be worth more than the sacrificed piece.