Guides are usually other beings who have incarnated as humans before. They are just like us accept they are not incarnated while they are your guide. They may or may not incarnate again on Earth but generally they have incarnated at least once before. They are fit to help guide you because they know what it is like to be here and they are not currently incarnated. They are essentially residing in their light body in higher dimensions and so they have a much broader view of reality. They are not distorted by the astral body and chitta so they can communicate with many levels, beings and dimensions clearly and with ease.
They have agreed to help guide you from the other side for this life. They are in contact with your own Higher Self and they know your purpose for incarnating on Earth. They know a lot about you and your future and will help in whatever way they can for your highest good. However they cannot help you if you do not want them to or if what you ask is against your Higher Self's purpose. All beings in higher dimensions respect your free will and it is a rule that they may not impinge upon. If you ask them to help you they will certainly do all that they can (as long as what you ask for is within your highest good).