Ive noticed most of your questions are basic things that you can look up anywhere, and id assume that the answers would be in the newbies section on this site.
Anyway,Visualisation is a focusing technique which allows you to generate and control the energy that you are using to cast your spell and magick your magick.
basically visualisation is thinkning, imagining and seeing your magick work which then stimulates energies which then will fullfil that desire, changing it from imaginative to reality.
Ways of doing this, is just plain day dreaming meditation. Picture in your mind a scene, such as a beach, make it very vivid like as if you were there, think of the sounds and fellings and senses that would be happening, then you need to control the scene, add some birds, waves, people, and so on, basically this trains your mind and will make you a more vivid and effective at visualisation^^