Its a really broad topic because a "spirit guide" can really be so many things. It can be a real live teacher or spiritual advisor. A guardian angel, ancestor or elemental. It could be an animal spirit you share a kinship with. A spirit guide could be a god form that you like. Or even a patron saint. You could use any number of mythical creatures or totally invent your own spirit guide.
Since spirit guides can be so many different things there are also just as many different ways to get into contact with them. A simple prayer to a saint, for example. Or something more elaborate like summoning an elemental.
I would say that unless you are experienced with magick it would be better to keep things as simple as possible. In my experience what has really helped me is to pay attention to my dreams. If you can recall your dreams then sometimes you can remember strange people or animals who seem to sort of guide you through your dreams. I would consider these to be a form of spirit guide. Sometimes you can call a spirit guide towards you. Many native Americans have rituals meant to reconnect them with their ancestors. Some are simply called forth by repeating the name of the ancestor before a ritual fire.
If you have a particular animal that you are drawn to you can surround yourself with pictures, scuptures even actual parts of that animal (like a feather or something, lets not go head hunting or something (IMG:
style_emoticons/default/wink.gif) ) But by surrounding yourself with this particular animal you are attracting their spirits to you. And you do not even need any rituals or spells or what not to do this. In addition to having things around you that attract your spirit guide, you can also invoke the spirit you seek by wearing things related to it. So say your particular animal guide is a crow or raven then you could fashion yourself a necklace use feathers or even claws. Then by wearing that necklace you will embrace the nature of that spirit and adopt its abilities.
Regardless of how serious you get into summoning spirit guides keep in mind that these guides are like friends. Like friends, they sometimes come and go when your lives end up taking different paths. Sometimes a spirit guide will teach you all that it can and will move on from you. You may even have multiple guides at one time. And if you ignore them, they may choose to leave you. Don't expect them to be good friends with you if you are not with them.
Every once in a while you may find a guide who becomes more than a guide and more of a permanent part of your life. I call these spirits familiars or kinship spirits. These entities don't simply guide and teach you, they become a part of you. When you hurt, they hurt and visa versa. And through this bond the strength of your magick, in my opinion, becomes stronger as well.
Also keep in mind that many times you already have spirit guides you simply have not learned to listen to them. This can be accomplished through dreams as well as altered states of consciousness that can be achieved through simple meditation. If at first you do not have success contacting a spirit guide, keep trying. Sometimes they speak to us in a way that can only be heard subconsciously until we learn to open that barrier and really hear what they are saying. A good spirit guide will never simply abandon you. But again, if you ignore them they may learn to ignore you too.