Can I join a team if I do not visit a city?

Based on this and this, you actually do need to go to a Pokemon Gym in order to join a team.

As for gyms, they only allow you to get Pokécoins without paying for them. Theoretically, you could play without encountering gyms (or Pokéstops) but you will most likely end up having to pay for pokéballs to catch things and lucky eggs if you want to level up as fast as possible. Unfortunately, Pokémon Go was developed with urbanites and city dwellers in mind. Playing out in the country just won't be as good of an experience as it stands currently.