If you go strait to the net and are playing some that is good they will kill you right up the line almost 75% of the time.
When someone like Montanes or Rochus S&V's every time they lose. But Karlovic wins every time, trust me height is good.
Sorry Glab, Dougie is right. You have to make up your mind before you hit the serve whether you intend to serve and volley. You may get away with 'gauging the return' at the level you're playing at at the moment, but once you get older those returns will be coming at you at twice the pace, so this tactic simply won't work and you'll be a sitting duck. I hope you take this advice, as it's not good to pick up bad habits and it's hard to drop them whilst you're in the development stage in the juniors.
Rochus won't serve and volley because his serve isn't effective enough, Karlovic will every time because he has a huge serve due to his size. Sounds to me like you have a good serve too, and tall for your age, so serve and volley will work well for you, and other juniors will be surprised by your tactic which can only be a good thing. Remember as you're coming in, take lots of small steps, then that moment of 'gauging the return' should be the point at which you split step and then anticipate the next shot. Commitment is the word you need to remember though. Commit to the serve and volley and even if you get passed, keep confidence in the tactic and that your opponent can't keep coming up with those shots every time.
Obviously if you find yourself a set and a break down might be time to change up the tactics and stay back, but until then, have faith in your aggressive style