My Items storage is often full with Potion, Super Potion, and Revive, and I cannot get more Poke Ball even when I am near a PokeStop. Which items should I throw away or use first, if I don't want to keep upgrading my storage and haven't gone to the Gym much, so I can get more Poke Ball?
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.
I usually keep all the balls, all the potions(I'd rather use a super and a small for 70hp than 2 super potions and so on), 15 revives max(though I could probably go well with 5 or so). I should probably mention that until lvl 13, I kept having to throw away revives or razz berries, and don't regret any of them. I did however throw about 50 pokeballs at around lvl 12-13 which was not a good idea. Also, I fight a lot so that's why I keep even the small potions.
The items you keep depend on how you play. Someone who hangs out at a park and constantly contests a gym is going to want every revive they can get. On the other hand, someone less interested in gyms won't have a need for revives or even potions.
If you're out of item space and have lots of potions/revives, take 20 minutes and duke it out at a gym to thin out your supply. You'll get a decent chunk of xp, and if you manage to take over the gym, you can claim a defense bonus for stardust and pokecoins.