Wanna is simply short for "want to". In casual speech, people don't bother to enunciate "want to", as it is overly lenghy. They prefer to simply say "wanna", as it is shorter and quicker to say.
For example, 1. I wanna buy a car => I want to buy a car
PS: Ferdis, sorry, when I began to write a responce, your reply was nowhere in sight
Just to clarify: 'wanna' is not really slang; it is just a written transcription of the casual native pronunciation of 'want to'. It should be used only jocularly or when representing the spoken language, as in dialogue.
And just to repeat - it has NO PLACE in a forum dedicated to learning English in posts like "I wanna no if dis is correct" or "I wanna rite my gf a ltr. help meeeeee"