They are lazy words used to replace "want to" and "going to". They are heard in very informal spoken English, so don't write them. Try to use them yourself in these sentences: "I'm going to have a beer", "I want to watch a film". I wouldn't recommend using them except with friends.

Honestly, please don't use them. If you still have a limited vocabulary and some grammar rules and phrases are beyond you, then adding slangy forms like "wanna" and "gonna" will simply make your English sound worse. You'll seem like a lazy learner who'll never speak English well. The other issue is that when learners try to use these forms to sound "authentic", the learner's non-native pronunciation makes "wanna/gonna" sound childish. Not to mention that the chances of you making grammar mistakes suddenly become much higher (yes, slang still follows grammar). At the very least, never put them in your own writing.

This website explains it http://blog.myhappyenglish.com/2011/09/21/english-lesson-want-to-want-a-vs-wanna/ http://blog.myhappyenglish.com/2011/09/19/english-lesson-going-to-vs-gonna/