Not everyone believes in a spirit guide, there is not proof they exist, so it's not necessary to have one, unless you want to
A spirit guide is something spiritual (means not physical) which somehow guides you or teaches you things. They can help you with problems, give you insights about yourself, help you in your spiritual healing process, help you in a physical healing process, can teach you spiritual technics, give you energy and so on. Really they can do a lot positive things.
They can come in many forms, human, human-shaped, an animal, a plant, some energy form, just a voice, really anything you can imagine.
And you can contact them in many forms. Most common is Journeying, Trance, Channeling, Meditation, Dreaming and the like, I guess. But there will me more ways to contact them. Again, probably any way you can think of.
I've got three animal guides (only two of them I'm in contact with right now) and I had human-like guide.