Pokémon GO presents, like any other free to play game, a shop and a relative currency that can be purchased with real money. The latter is represented by Pokémonete. The items that can be purchased with these coins range from simple Pokéballs, to aromas, up to the lucky new ones (read here how to use them to level up) and to the expansions for the inventory.
The prices are surprisingly low and honestly very inviting, but fortunately ignorable: among the various game mechanics (which the title does not explain) there is one that will allow you to acquire Pokémonete for free.
The "trick" is very simple and to implement it, all you have to do is conquer gyms. Opening the shop in game you will notice this symbol:
That shield icon will indicate the number of gyms you are currently monitoring. By tapping on it you will receive a very generous prize: for each gym you are checking you will in fact receive 500 Stardust and 10 Pokémonete.
But be careful to take advantage of this possibility, as once used it will have to recharge for 20 hours.
Other Pokémon GO guides:
Pokémon GO: Basics Guide
Pokémon GO: How Eevee evolves
Pokémon GO: How to level up quickly
Pokémon GO: How to save battery and traffic
Pokémon GO: Everything you need to know about stardust
Pokémon GO: Guide to weaknesses and strengths
