I'm making a 3D tetris game and i want the sides of the Gridbox that the blocks fall into disappear when looking at them, that way the player can see his blocks going in. Of course i don't want to make all the sides see thru, just the ones i'm looking at. How to do that with blender? Or is it not done in a 3d program but in the actual programming of the game?

Make the faces point inside out. Polygons are 1-sided (usually), so just make a box where the faces point inwards instead of outwards.

Thanks that's exactly what i needed :)