Milky, for the base of your terrarium, on top of the gravel, put down some activated charcoal to help keep the soil free of excess nutrients. The gravel and charcoal should come up to about two or three inches, so you can see where the water level is in the tank. On top of that, a mix of equal parts peat moss and sand will be good for most CPs. A good way to distribute the soil is on a slope, with the front of the tank having less soil than the back. This way, you can see all your plants better, and plants that like wetter soil can be placed in front, where they'll be closer to the water table.
Good luck!
~Joe