You can nickname your Eevee and get the generation 1 or 2 evolution you want.
Name your Eevee exactly as in the table, with a capital letter. Once you evolve it, your Eevee will have your chosen evolution. Please note that you can use this trick only once per evolution!
Eevee are still quite common and evolve in excellent attackers or defenders, so I recommend to catch a lot of them! In the current meta, the most useful evolution is the Espeon, since it is needed to beat a Machamp during a raid. A few level 30 Espeon will do the trick. Machamp are currently considered the best attackers to beat gyms.
Here is the list of Eevee evolutions!
Eevee nickname evolutions
You can also evolve an Eevee to Espeon or Umbreon without the above naming trick, by doing the following (be careful, do exactly as documented):
- Walk at least 10 km with the Eevee as buddy
- Wait until you get the candy
- Double check that you have no GPS issues; Pokémon GO relies on GPS position to determine whether it is day or night time
- Evolve the Eevee from the buddy screen
- If you do this during day time, you get an Espeon, otherwise an Umbreon.
Note that contrary to the naming trick you can do the above multiple times.