Could you please tell me some examples for formal and informal talk?

An example of formal talk would be to say in a greeting "Good morning" while informal would be to say "Heya!" or "Hi!" When you think of formal and informal, think what impression you want to make to the person - do you want them to think you're being serious? or professional? or are you talking to a friend, where more relaxed language would be more appropriate. Or when writing something, do you want a professional feel to the work (which would be formal) or a more conversation tone? (which would be informal)

Another example if you want one is this answer - i have written it formally just for you.

Informally would be "Uhm, well just think about it!! Or look it up in dictionary.com? :)"

hope this helped XDD