We're getting deeper into the realm of colloquial speech and individual opinion, Manohonor-- there are not such rules for what you can say, as there are for what you should write.
(1) To me indicates a lack of a proper education and upbringing. Of course, the extreme reduction of the initial /hwuch?/ may in fact represent 'what are you'. The full sentence should, in any case, be prope
How closely you imitate this kind of speech depends on the people you are interacting with. It also depends on how good your English is, even on how good your pronunciation is.
If you don't mimic the sounds exactly and make them "sloppy" in the "right way" -- the way a native English speaker would instinctively do it, then as a non-native speaker, you will probably not sound very authe
Use the full versions in writing and in formal speech. The others you should perhaps think of as verbal shortcuts - those who know the territory well might nip down a shortcut rather than take the offical path but it is easy to get lost!
You will never be wrong if you use the fully correct version.
In addition to the fabulous advice you've received already,
1)What you looking at? ('are' is necessary for Standard English, which isn't to say that omitting 'are' is incorrect for other dialects of English)
Spoken forms: What are you looking at? What're you looking at? ('what' and 'are' contract; the apostrophe (') represents 'a' of 'are') Whadarya you lookin