Save, Save, SaveIf you are going to be successful at Pokemon GO, and if you are ever going to stand a chance at finding all 386 Pokemon currently coded within the game, you must learn fast to save your Stardust, Pokemon Candy, and items. Do not waste valuable Stardust and Pokemon Candy on low leveled, unevolved Pokemon - unless, of course, to evolve said Pokemon to increase your pokedex or gain xp - but wait until you have a fully evolved Pokemon (Final Stage) before using them. This will allow you to save as much of them as possible, as well as give you the most to work with when leveling up said Final Evolution into a powerhouse.
And the same reigns true for your items. You only get a specific number of slots in your bag - unless you make the decision to purchase more space - so try to save and fill that bag as much as possible. You never know when a bunch of Potions or Revives will come in handy at a certain Gym or during a certain Raid Battle, or when you will need tens of PokeBalls of all sorts to catch a rare, difficult-to-catch Pokemon.