Heyyyy

Hey, how are u doing?

It's my second note and I'll try to write it right, ok? :))

But u must promise to me that you'll correct my mistakes. Ok, now I'm gonna tell you something about myself. I've started to learn English for about 3 months ago, so now I have some questions now. At First, what is the different between "Ok, I understood it" and "Ok I got it". Why do some people prefer to use phrases like "pick up", "get it", "check out" and e.t.c. but other people prefer to use usually different phrases.
Well, that's all. I'll write one more note next time. Please feel free to answer to me.

* You can use "u" insead of "you" but it comes off as very, very colloquial and - depending on the context - uneducated. It's fine for texting or an e-mail to a friend, but beyond that it's seen as lazy and/or immature. Same thing with "gonna" versus "going to."

*Make sure you have a space after a period or comma.

*Oh, and smiley faces get eyes :))

As far as your questions:

There is no literal difference betwen "Ok, I understand it" (you can say understood, but it implies that you might not understand it now) and "Ok, I got it." "I got it" is more colloquial - it's slang, whereas "I understand" is more formal

And for the other phrases, it just depends on who your're talking to, and the type of person you are. Sorry I can't give you a better answer. :(