Watch this short video to get some pointers on foot positioning and climbing on board a SUP from the water. Simple techniques that can aid your progress in learning how to SUP.
Once you stand up, you will want to have your feet approximately shoulder width apart with your toes pointed towards the front of your board.
If your board comes a carry handle, you can use that as a reference to the center of the board which is where you'll want to have your toes approximately, while you are standing on the board.
Climbing onto a board is very similar to climbing out of a pool. You will want to put your hands on top of the board lift yourself and pull your body up onto the deck of the board.