Eevees Are The Way To GO
One of the wisest things any Pokemon GO player can do early in the game is catch plenty of eevee. For one, Eevee is usually not that hard to catch. And the more Eevee you catch, the more Stardust, Eevee Candy, and xp your trainer will gain. Eevee also has a high appearance rate like pidgey and geodude (depending on your geographical area), allowing you a better chance at catching more of them. Again, the more, the better.
Another reason is because eevee only requires 25 Eevee Candy in order to evolve. This is an extremely low cost to gain a Final Stage evolution, which is when the best trainers begin training their Pokemon. After evolving, the Eeveelution gains an enormous increase in Combat Power! This is so greatly beneficial to trainers, as you go from having a little Eevee into an instant powerhouse!
Finally, eevee has so many possible evolutions, including vaporeon, jolteon, flareon, espeon and umbreon. eeveelutions are at random when trainers do nothing to their Eevee, but players can nickname their Eevee a particular nickname to evolve them into their choice of Eeveelutions (but just once). Even without this trick, players should catch as many Eevees as they can in order to evolve them, since you honestly never know what you will get!