Make sure your eyes are focussed on the horizon.
Remember to keep the aircraft where you want it, don't let it pitch up or down whilst trimming.
As far as which way to trim, imagine you have a cannon ball in the aircraft. If the aircraft is out of trim, where is the cannon ball? Pushing forward = heavy tail, cannon ball in the back. Pulling back = heavy nose, cannon ball in the nose.
You want to move the cannon ball into the middle of the aircraft. Using the trim wheel, move the wheel in the direction you want to 'roll' the cannon ball. You need to relate the movement of the trim wheel to the decreasing pressure on the control column. Move the cannon ball too far and you've gone too far!
Lastly, don't hold onto the controls too tightly. Flying your C152 is all about what you see out of the window and what you feel on the control column.
I hope that makes sense. I would suggest telling your instructor that you want to work on trimming, might be worth flying S+L for a while, each time the instructor giving you the aircraft out of trim at various extents, sometimes grossly out of trim, sometimes very slightly.
Please note:Don't actually put a cannon ball in your C152, goodness knows what it would do to the W&B. No cannon balls were harmed in the creation of this post.