A spirit guide is a form in the spirit or astral plane that's main purpose is to help and guide those on our plane. They come in many forms, the kinds you'll hear the most are angelic or animal guides. To contact them, there are many ways to do this. The most common is through a series of meditation. The way I made contact with my spirit guide was by finding a quiet place, relaxing, and imagining myself breathing in a calming positive light. Once my body was filled with the light I would let my mind wander through a woodland or nature like area that my mind alone had conjured up. I would imagine what I felt at the time, the sounds and smells, till I'd come to a point in The road where I'd see a shape or form. At this point you will call out to him/her and ask them to reveal themselves. Of course this isn't the only way but A way.
Blessed be and good luck.