The moment where the game begin is after catching yours first pokemon. Remember to catch'em all!
Now at the begginning I'll show you basic info about the game, the most important things you need to know.
Pokestops are places on your map, monuments or some special places in the world downloaded from google maps. When you're near those Pokestops you can reedeem items every 5min for each Pokestop. This is the best option to get pokeballs!

Thats how Pokestops looks if you'rnt in their radious (Those blue points on my way)
When you'll reach Pokestop the animation of it will change to one like on this picture.

Then if you press on it you'll see what is in this place. And after spinning this picture token you will recive your poke items.
Used Pokestop will change they color to purple till they will recharge.

Remember to choose the most possible way with Pokestops on your poke walk.