What should I evolve my Eevee into? I think a Flareon would be good, since I have Wartortle and Pikachu as my Water and Electric types, but I also think a Growlithe might be a good Fire type. I only get one Eevee, so the choice really matters.

Flareon sucks. Physical Fire type. Decide on Espeon, Umbreon, Leafeon, or Glaceon, depending on what you need more.

Espeon, Umbreon, Leafeon, and Glaceon do not exist. This is Pokemon Blue I'm talking about, thought that was made obvious by the "only one I'll get" comment. No breeding, no Physical Fire.

1. I'm not even going to ask why you're playing Blue.
2. Flareon still sucks. It's got more Attack than Special Attack (Or just Special, as the case may be).

1. Any Fire type pre-R/S/E not named Ninetails, Magmar, or Houndoom are crap, not counting Legendary birds. They're ALL physical attackers! In Blue, if you're desperate for a Fire type, just catch a Vulpix en route to Erica's gym, and call it a night!

2. Evole to Flareon only if you're filling a pokedex. IIRC, Flareon's the physical one, Jolteon's the Special one, and Vaporeon's the Defensive one. Flareon is surpassed by Charizard (which I see you don't have), Arcanine, and Rapidash; Char for pure strength, Arc and Dash in speed. Another reason not to pick it.

3. Eeveelution is just for novelty, especially in R/B/Y. If you're seriously looking to use one to fight with, pick Jolteon. Owns Pika in speed, yet slightly behind in Sp. Atk.

I think I'll make it a Vaporeon, it's better than Blastoise and I like Water types more.