look for players with expired contracts. then the position you need. then certain key attributes you need for that role, eg, for a striker, look at finishing.
if there are only a few players listed there, track them all one by one. if there are many, look for another important attribute you want in that player, say, first touch, or off the ball, etc etc..
for a CB, marking, tackling, positioning are key.
for GK, handling, one on ones, and reflexes are key.