To level up as a trainer you must earn experience points, XP. And the fastest and easiest way to earn XP in Pokémon GO is...making new friends!!. Do you want to know why?
Let's compare Pokémon GO main actions and tasks: Turning a PokéStop will give you 50PX (also you need to be close to one and wait several seconds to be able to turn it again); capturing a Pokémon will give you 100XP if you already had one of the same species (also you have to wait for it to show near-by); and evolving a Pokémon will give you 500XP (also you have to have the required Candy to evolve it).
On the other hand, adding a new friend and exchanging a first gift (which will get the friendship to Level 1) will give you... 10.000XP!! That's the equivalent of turning 200 PokéStops, capturing 100 Pokémons or evolving 20 of them. And the only thing you need is a trainer code, that you can easily find on the homepage, and send a gift. In less than a minute you'll easily earn those 10.000 XP.
The importance of leveling up
To progress in the Pokémon GO game you need to take actions to earn experience points (XP) that will eventually level up your trainer. You start the game at Level 1, and you can progress up to Level 50. Advancing towards new levels is key because as you level up you unlock new game features and improvements, and the Pokémons you capture are potentially stronger.
It is relatively easy to level up on the first levels, and you will achieve pretty soon. As an example you only need 10.000 XP total to move from level 1 to level 5. But as you progress this becomes more complex and is more time consuming. As an example, to get from level 39 to 40 it takes 5 millon XP.