Instead of repeating a noun — Do you know Marcos? Yes, I know Marcos — you use a pronoun: Yes, I know him.

	Italian	English
io	mi	me
tu	ti	you
lui	lo	him, it (masculine)
lei	la	her, it (feminine) — and polite you: La
noi	ci	us
voi	vi	you (plural)
loro	li / le	them (masculine / feminine)

For things, choose by the gender of the noun: il libro → lo, la pizza → la, i biglietti → li, le chiavi → le.