Where can I find Pokestops
You can find Pokestops in your game by looking for blue cubes on the map. These can normally be found at areas of 'significance'. Examples are remarkable buildings, statues, playgrounds and art.
By visiting Pokestops and "spinning" the Pokeball marker, the Pokestop can drop a variety of items. Players will want to travel to Pokestops as often as possible to gain XP and collect items such as Pokeballs, Eggs, Max Revives, etc.
How can I interact with a Pokestop
If you are not in range but can see the pokestop on your screen, you can tap it to interact to see the name and the picture of the pokestop. "This pokéstop is too far away." text will show when you are not in range.
Around your avatar you have a purple/white range indicator, when you are close to a pokéstop you can interact with, the cube on the top of the pokéstop might change to a pokeball-like icon. When you can collect items, there is no special text to indicate for that.
Pokestops are blue when you can collect items from them and will turn purple when you've got to wait. It takes around 5 minutes (can be different each time) for a Pokestop to go from Purple back to blue.
You can collect free items from a Pokestop. Simply go to the Pokestop, click on it and if you're close enough you can swipe the circle with the picture. The pokéstop needs to be in range when you swipe or it will not give you items. Items appear in individual bubbles. You can pop these bubbles but you don't have to as closing the window will also give the items to you.
Normally pokéstops give you 3 items and 50XP. However you can get more items if you are lucky. Collecting 6+ items from one swipe gives you 100XP instead of 50XP.
You can place a Lure Module on a pokestop to attract wild pokemon, these will benefit all players within range of the stop. This will last for 30 minutes until it has run out. To place a Lure Module on a pokestop, press the white bar above the picture on the stop. You can now select the module to install. You will know there is a lure module in place if there is what looks to be pink petals raining around the stop, this means another trainer has used a lure module.
Creating a Pokestop
Pokestops are added by Niantic and at the launch of Pokemon Go they were based on the user submitted portals of Niantic's previous game, Ingress. You can request to add a new Pokestop here. You will need to enter the name, address and upload a photo. If your request gets approved then it could become a Pokestop or gym.