someone or somebody?

What’s the difference between when using ‘someone’ or ‘somebody’ a conversation?

Usually there isn’t one. They are interchangeable.

I find something interesting. What about the negative?
“I am nobody.” OK.
“I am no one.” Is this OK?

Yes (but less common in the context you give)

“Nobody wanted the party to end.”
“No one wanted the party to end.”

‘Everyone’ and ‘everybody’ are usually interchangeable too.