Will you be using a room in your house, the garage, or an outdoor space? You may only have one choice here, but here’s some things to consider as well.
If you are using a room inside your home, the ground floor is recommended. Weights even gently placed make noise. Jumping and ball slams will not be well received either to anyone below deck.
How Much Square Footage
You can make a gym work in even the smallest of spaces, but you'll want some dimensions as you'll have to plan for how much space each piece of equipment takes up as well as the space required to store this equipment.
Sketch It Out
You can get crazy like I did and use the free architecture software Sketchup, but really, a simple hand drawn sketch will work fine. As you pick your equipment, you can add to the sketch. This is much easier than moving equipment so take the time to use whatever art skills you have.