It's kind of a broad question, but the short answer is to set limits and boundaries, and enforce them. Bit like parenting, really.
For example, if she pushes into your space, drive her out. Back her away, or yield her forehand/hindquarter (whatever portion of her anatomy is in the way) away from you. If she won't respond to your body language (hard eyes, raised energy, pushing her with your hands), you may need a dressage whip or NH-style stick to back up your request. Once she moves her feet out of your space, go back to doing what you were doing.
Take no crap, but be fair. Neither nag nor beat.
I assume from the phrasing of your OP that you're working with a trainer/instructor? If so, he/she should be able to guide you in person, and give you some more specific insights. If not, and your difficulties escalate, I recommend enlisting the help of a more experienced horseman to help you out.
Best of luck!