If you want to fight at gyms you’re going to have to level up your Pokemon, so that means increasing their CP stat. You can do this in the power up section when you are looking at your individual monsters – a combination of Stardust and Candy will do the trick. You can get Candy when you send back that specific Pokemon to Professor Willow so keep an eye out for that specific Pokemon and keep sending them back to earn more. It’s big numbers of Candy that’ll let you evolve too.
When you evolve your Pokemon, you want them to be as high a CP as possible in order to evolve into an even more powerful beast. If you want to get into the nitty gritty of exactly how much you can level up, pop your Pokemon’s details into an IV calculator such as Pokeassistant. This breaks down every category to give you information on your best possible ways to evolve.