1. Capture Pokemon
Capturing Pokemon is one of the best and most efficient ways to gain XP. You will gain experience points for skillfully throwing Pokeballs (Eg. 110 XP for nice throw, 150 XP for great throw, 200 XP for excellent throw) and capturing wild Pokemon.
Not only will you gain a set amount of XP for capturing Pokemon, if the Pokemon is new you will get an added XP bonus (500 XP)! Even better, this added bonus is substantially more than the standard XP award you get normally. Capturing new Pokemon is most effective for lower level players, as the XP requirement for each Level up is much less in the early stages of the game.
2. Hatch Those Eggs
Carry around those Eggs you find at Pokestops, walk a total distance of about X Kilometer depend on your Egg, your Egg will hatch into a Pokemon. You will gain XP for hatching the Pokemon (200 XP for 2 Kilometer Egg, 500 XP for 5 Kilometer Egg, 1000 XP for 10 Kilometer Egg), regardless of the species and if the Pokemon happens to be new you will gain the added XP bonus (500 XP) for obtaining the new Pokemon!
3. Evolve Pokemon
Capturing Pokemon will award you with Candy. Each species of Pokemon needs a specific amount of Candy before it can be evolved.
Acquiring Candy for Evolution can take some time, then you will get the XP reward (500 XP). Make sure to try and evolve as many Pokemon as you can. You will not only strengthen your Pokemon team, you will also be rewarded with a great deal of XP.
4. Train & Battle at Gyms
Now Gyms are not as easy to find as Pokestops, but when found are great areas for gaining XP. Do your best to find friendly Gyms, especially if you are a newer Pokemon Go player. Friendly Gyms allow you to train your Pokemon without having to deal with the (usually more powerful) opposing Teams’ Pokemon. Your Trainer will gain XP for each defeated Pokemon.
If you stumble across a Gym controlled by another Team, you can still battle, but winning these battles is oftentimes more challenging. Check the Gym first to see which defending Pokemon you are up against. If you have the CP and/or Pokemon Type advantage go ahead and initiate the battle. You will gain XP for each Pokemon that you defeat!
5. Visit Pokestops
As you travel you will find a variety of Pokestops to visit. These locations will give you Pokeballs, Revives, Eggs, etc. Most importantly, visiting Pokestops will give you 50 XP. You can visit a lot of these in a short period of time and the XP really does add up, especially for beginners with lower Trainer levels.
6. BONUS TIP (Gain 1,000XP per minute!):
Collect as many Pidgeys, Caterpies and Weedles as you can. Next make sure you have a Lucky Egg (With doubles XP for a short period of time), which you will get as a Level Up fast in pokemon go. You can also purchase a Lucky Egg from the Shop for Pokecoins.
The reason you want to focus on capturing these three Pokemon is that they are easy to find/capture and only require 12 Candy each to Evolve. Once you have collected enough Candy for each of these Pokemon (try to get at least 15 evolutions worth) you are ready to Evolve them.
Use the Lucky Egg and immediately begin evolving those Pidgeys, Caterpies and Weedles. You will gain a lot of XP for the evolutions plus that XP will be doubled because you are using the Lucky Egg buff!