It's 3D and i'm using lwjgl (OpenGL) and I only want to do really basic stuff. I had an algorithm that worked but I couldn't detect where I walked into the cube. The algorithm was like on collision x -= x - sizeX (same with z), but since I cant detect what axis i'm hitting then cube on then it brings me to the same spot.
You have two choices - you either find the minimum axis to move out along (i.e. the axis along which the intersection is smallest) or you take advantage of the fact that you know the direction the objects were moving to decide which way to move out.
If you are not using the direction the objects are moving or the previous positions of the objects, there is no "correct" direction to separate them by - imagine the objects come into existence intersecting, there is no correct way to separate them except along the minimum distance, which is commonly used in physics simulations.