[Phassetus]

Or should I just defend and block from the start?

Even if you are a beginner some referees may be eager to award you penalty for fight avoidance or award a point to the other person even if his/her attack is not perfect.

So in opposiotion to your sensei I would advise to attack, attack and attack. This way your opponent's attacks will be disrupted and you will have a chance to score.