Idiomatic expression

I'm sorry I would like you help me to understand an idiomatic expression or I think that it is, I'm talking about I got and I've got I don't understand how I can use this

Could you give me an explanation about to use it

Thanks a lot

[Mister Micawber]

These are not idiomatic expressions, Rigoberto, they are just two forms of the informal stative verb 'have got' = have. The first, 'I got' is substandard, and the second, 'I've got' is informal, for 'have' (= possess), as in these sentences:

'I have a headache.'
'I've got (have got) a headache.'
X 'I got a headache.'

The forms have other uses, however, as the past tense and present perfect tense for the active verb, 'to get':

'I got a letter from my friend yesterday.'
'I've got (BrE)/gotten (AmE) a letter from her every day this week.'

I am a bit confused here, too.
I always hear/see sentences like this:

I gotta go.

according to some dictionary, 'gotta'= 'have got to'

Don't understand why people would say 'I have got to go'...Isn't that "present perfect"?

thanks

[Mister Micawber]

Now we are into a third form-- 'have got to'-- which is a modal idiom. It means 'must', in the sense of necessity or requirement. It is not present perfect; it only has two tenses (and three forms)-- have/has got to, had got to.

'Gotta' is a common transcription of the casual pronunciation, which is usually pronounced with a 'flap-t' (= /d/), actually: /gadə/