Powering up, also known as levelling up, is quite simple and will increase the combat and health points of a Pokemon of your choosing. Powering up is important because it allows your Pokemon to launch stronger attacks. They will also be able to last longer during intense gym battles. Unlike previous versions of the game, Pokemon do not level up after battles. Instead, you need candy and stardust to power up. The process of doing so is quite easy. From the map view, select the Main Menu (Poke Ball in the center of the screen) and tap Pokemon, then select the Pokemon you want to power up. From the subsequent screen containing the stats of the Pokemon, tap power up.
How To Powerup Pokemon
In Pokemon Go, you need a specific amount of stardust and candy to power up a specific Pokemon. You can get the exact figures by viewing the stats of the Pokemon you wish to power up in your collection. Usually, it requires a lot more stardust than candy to power up. Therefore, you need to focus on collecting stardust whenever possible. There are various ways of getting Stardust the most effective and common being catching wild Pokemon. At the early stages of the game, every Pokemon caught will earn you at least a hundred Stardust. Stardust can also be obtained by hatching eggs and defending gyms. Defending bonuses will include Pokecoins and Stardust. 
Candy, unlike Stardust, needs to be of a specific type in order to be used to power up a particular Pokemon. You get the Pokemon candy by collecting the same Pokemon you want to power up repeatedly. The best way to catch the same Pokemon repeatedly is by visiting suitable habitats for the specific Pokemon. Grass types can be found in grassland areas while water types will appear near bodies of water.
When To Power Up In Pokemon Go
Powering up can be done basically whenever you want once you collect enough resources. However, powering up uses up a lot of your hard-earned candy and Stardust so you need to make sure that it will not be all for nothing. Therefore, it is important that you evolve your Pokemon before powering up. You should make sure that the Pokemon you are powering up has good moves that will do a lot of damage during gym battles whether in defense or attack. 
Powering up Pokemon will not change the moves they have, so you should not bother powering up a Pokemon that has any mediocre moves like Tail Whip. Evolve your Pokemon early so that if they end up with weak moves, you can save your candy and stardust to use on other Pokemon. It can take quite some time and effort to get a Pokemon with good moves, however, do not give up because finding the right Pokemon is the key to success. It is also more convenient to power up after battles so that you replenish any lost health while also making the Pokemon stronger.
Should I Power Up My Pokemon or Evolve Them?
When you're trying to get stronger Pokemon, you might be torn about whether to evolve them or power them up. The good news is that no matter which order you do these in, your Pokemon will end up at the same CP. If you evolve then power up a Pidgeotto, it'll be the same CP as if you had powered up then evolved. So don't panic about doing these in the 'wrong' order -- there is none! 
However, if you have a choice between powering up a Pokemon and evolving it, you should evolve it. Evolving nets the trainer 1,000 experience points, while powering up nets you none. If your trainer level goes up, you can catch higher CP Pokemon without having to waste stardust. The generally agreed upon time to start powering up your Pokemon instead of just evolving whenever possible is around level 20. At level 20, it becomes very difficult to catch Pokemon because of their heightened CP. It will then be easier to power up than to catch. Don't worry if you've already powered up low-level Pokemon; you'll still get a chance to catch high CP Pokemon once you level up more! Also, if you put Pokemon in gyms, you'll get stardust each day as a gym reward. This can help you gain back any stardust you've used to power up your Pokemon. Keep on keeping on to make your Pokemon the strongest in your town or city!