The most important thing you can do in "Pokémon GO" is level up your Trainer. The higher the level you are, the better Pokémon you'll find. They'll have higher CP and HP, and thus they'll be more capable of going to gyms and winning in battle without you having to power them up.
So, how do you get there quickly? Be recycling your duplicates. Pokémon like Rattata and Pidgey and Weedle — Pokémon that you run into frequently, that cost very few candy to evolve. Evolving Pokémon gives you a fat payout of 500 XP. You see where I'm going with this?
Every time you collect a Pidgey or a Weedle or whatever else, you get a few candy. When you evolve them, you can then transfer them to Professor Willow for a bonus candy, thus enabling further evolution.
Here's a bunch more info!