You say “hey, sexy girl” in Spanish like this: Hola, sexy chica!
The word "sexy" means exactly the same in Spanish as it does in English, so you do not have to worry about learning an entirely new word just to call someone sexy.
The example above is the most straightforward way of calling a girl sexy, but there are other phrases you can use too. You could say “hola, preciosa” which means “hey, beautiful” or “hey, precious”.
The option you use will probably depend on who you are speaking to.
For example, if you wanted to call a *guy* sexy, you would say “hola, guapo”, which basically means “hey, handsome”. Sexy wouldn't be as common when talking about a guy...
You never know when you might need to use these phrases, so it is always good to have them handy in my opinion! Also, if you want to learn to say “I love you in Spanish”, or even “hey, what’s up”? Blurtit will be able to help you out.