I Am Tiling A Shower Wall But My First Two 12" X 12" Tiles Want To Slide Down. Any Suggestions?

Not sure what you mean by "first" two tiles. Are you starting from the middle or the top?
I usually nail a thin strip of wood on the wall as a ledge to rest a row of the tiles, and to keep them perfectly level, then add rows above that. I then remove the strip after the tiles are set, and tile in the remaining space under.

Your tile is 12x12, are you using the right trowel? It should be at least a 1/4 inch "notched" trowel. "VERY IMPORTANT to have the right trowel for that size tile.This will create more suction between tile and cementboard, thus giving you a better hold to the wall. Home Depot has a display of the right trowel according to the tile size. Porcelain tile is heavier than ceramic tile so you want to use the right thinset. Are you using one that you mix yourself? If so, you will need to make it thicker but instead of doing that, you would be better off buying the premixed from H. Depot. They have a brand called "Gray" spread and set. It has a yellow number 1 on it . They also have a premixed "mastic" called AcrylPro that is a very good product. Both these products are nice and thick. When you lay each tile, wiggle it up and down a little using light pressure to ensure a good seal.  Backerboard and cementboard are two different boards. I am assuming that you are using the cementboard. You can use either product. But if you decide to tile a concrete floor, then use only the "Gray" thinset. If you are using backerboard, go with the AcrylPro mastic. Pick up some of the "round" tile spacers. They are a bit more expensive,"around $5.00 a bag", but well worth it. Set them in the corners of the tiles as you lay them side by side. They will keep your lines straight and will help you to see that the tiles are level with each other. On 12x12 I would use 1/8 inch spacers on the wall. 1/4 inch on a floor. This should solve your problem. If not, then you will have to start at the bottom of the wall and work your way up. Just make sure you have your measurements worked out and definitely using the spacers: Always remember your focal points on each wall. Lay the bottom row out against the walls before you start if you need to so that you can get an idea of how it will look.