It's all about reading the defense. If your defender is playing close, shake him off with a crossover and go hard to the board! If you doubt yourself or go weak you're going to be stopped. Work out and build muscle if neccessary. Watch clips of your favorite players and examine closely what they do and how they drive it to the hoop. My best advice is to be confident and be forceful. It also helps if you got some moves to throw off the center. Spin move, Euro step, power move, etc...
Well, work on your speed. Jump roping will help that, and sprinting short 20-30 yards .
Also , add some pump fakes, jab steps, head fakes, anything to create space. The moment your defender reacts, you should have a counter move to get by them . You see Paul Pierce isn't the fastest NBA player, but he knows how to score by having an array of moves .
Get stronger, quicker, and work on your ball handling. The more effective you are as a ball handler, the better your crossover and control of the ball will be. And it will make it easier to get into the lane, and score or pass to your big man .
I say watch CP3 or Tony Parker or Deron Williams on Youtube . Footage of their game shows what they do to be great point guards.
Also, you need to get a respectable jump shot. Once the defender starts guarding your shot, you basically control the defender from then on. They think you'll shoot, and you'll pump fake and get into the lane.
Last, get a floater or runner. You need to be able to get a shot off over taller defenders. If you watched Game 6 of IND/MIA you probably witnessed Dwayne Wade keep his dribble low to split double teams, and finish with a runner or floater .
Work hard .!