There are 2 easy (ish) ways to score in pro evo 6.
Firstly, if you can play a through ball to a striker making a run (and they are decent), you can quickly shoot and cancel with short pass (square + X), change direction slightly (diagonally) to go around the keeper, and just tap in.
Alternatively, get the ball to a winger when they are behind the opponents fall back (or just give it to them if they are quick) and get them to run towards the edge of the penalty area. If you have a man in space, stop running for a second and do a short pass. As soon as the ball is going towards the player in the box, just hit X and they should take it first time and its likely you will score.
The most important part of attacking in PES IMO is to use the radar to find areas where their defence has gaps and you have players. I would recommend starting playing on 2 stars until you can reguarly beat good teams, then start to move the difficulty up.