How do you say i know in spanish?

Depends...

Sé -- e.g. sé su nombre = I know his name

Conozco --- e.g. Conozco a Juan = I know Juan

Lo conozco = I know him

La conozco = I know her

Lo sé = I know (that concept/idea)

--- If you are just responding to some one saying something you already know, you would say "lo sé"

If "I know" is the beginning of a sentence the "lo" changes to "yo".

I know where José is = Yo sé donde esta José.

I know José = Yo conozco a José

If you want to say "I know him" then you use both yo and lo. "Yo lo conozco"

Conocer = to know someone

saber = to know something (item or idea)

Strangely enough if you want to say "I know who he is" you would say "Yo sé quien es." or "Yo lo conozco"(I know him)