Pokemon-go – How should I exit/close Pokemon Go

You need to have the PokéStop or Gym inside your "action circle" -- the pulsing circle that will sometimes spawn. This circle's radius at its widest point is 40 meters, so you must be within 40 meters of the building you want to interact with. This radius is set to the same as Ingress'.

When a building is eligible, you'll notice that Pokéstops will "expand" out, and the action circle will leave a trace mark on the ground.

If you're not using gyms at all you won't need Revives and Potions. You can safely throw them away. You only need Revives when losing battles at enemy gyms. Potions are needed for both, enemy and friendly gyms. So if you don't fight you won't lose a single hit point.

My advice would still be to keep a certain amount just in case you want to fight at a gym or just find an empty spot in a friendly gym.
I throw anything above 10 Revives away and keep only the best Potions. The 20 hp standard Potion can go down the drain, if you have a few better ones. I usually keep 20-30 Potions and get rid of the rest. This way you have a lot of space for Pokéballs without buying extra space.