Since Spanish has two forms of "to be", ser and estar, it's common for people new to the language not to know which verb to use or when to use one or the other when complimenting someone on their looks or personality. This post will clear this up for you easily.

Since estar is more for temporary situations use it when complimenting someone at that particular time.

If you want to tell a woman that she looks beautiful right now or today or this evening you can say:

You are beautiful. (You look beautiful right now)
Estás hermosa.
eh-STAHS ehr-moh-sah.

To compliment a guy:

You are handsome. (You look handsome right now)
Estás guapo.
eh-STAHS gwah-poh.

If you want to tell someone they are a beautiful woman or a handsome man, use ser instead:

You are beautiful.
Eres hermosa.
eh-rehs ehr-moh-sah.

You are handsome.
Eres guapo.
eh-rehs gwah-poh.