Gonna, gotta, wanna, gimme, gotcha are frequently used in speech in informal colloquial English, particularly American English.
You don’t ever need to use these forms actively yourself. They may sound too informal if you do, although if other native speakers of English around you are using them, there is probably no reason why you shouldn’t use them too. It is, of course, important to recognise and understand them.
gonna = going to, Note that in the interrogative, are is omitted in second person singular and first and second person plural – What we gonna do now? (= What are we going to do now?) / We’re gonna carry on and try and get there before dark.
gotta = [have] got to, You gotta / You’ve gotta get changed right away. The match starts in five minutes.
wanna = want to, Wanna can be used with all persons singular and plural, except third person singular (not with he/she wants to ) – What you wanna do now? (Instead of: What do you want to do now?)
a wannabee derives originally from the US, but is now used extensively in British English. A wannabee (literally a want-to-be) is someone who is trying to copy somebody else. Usually the person they are trying to copy is somebody famous. ( Scores of Britney Spears wannabees raided the shops where she had bought her latest outfit.)
gimme = give me,
gotcha = got you,
lemme = let me, letcha = let you,
outa = out of,
c’mon = come on,
ya = you,
yeah/yep = yes,
nope/naa = no,
betcha = bet you,
kinda = kind of,
lotsa = lots of,
lotta = lot of,
helluva = hell of a,
a = going to (i’m a = i’m going to)
ain’t = is not – Note, though, that “ain’t” isn’t limited to “is not”. Like gonna, gotta, wanna, it can be used with any person. – I ain’t, you ain’t, he ain’t, etc.
There are also Britishisms:
Cuppa = cup of tea
Pinta = pint of milk
Dunno = don’t know