How do you say, “You are beautiful” in Spanish?

“Eres hermosa”.
To a woman or girl, you say:
“Tu eres hermosa“.
Tu — means “you” (second person singular)
eres — means “are” (be- verb for 2nd person singular)
hermosa — beautiful
Or simply, we say:
“Eres hermosa”.
Synonyms for “hermosa” are: guapa, bonita
Other ways of saying it are:
“(Tu) eres guapa”.
“(Tu) eres bonita”.