You'd say 'Veux-tu quelque chose à boire?' to a friend, a family member, a classmate, or a child. Kids and teenagers would say this among themselves. To a stranger, or to an adult whom you don't know very well, it's polite to say 'Voulez-vous quelque chose à boire?'

"veux toi quelque chose à boire" is incorrect native French speakers would rather say "Voulez vous quelque chose à boire "

"Veux-tu quelque chose à boire" is correct. Honestly, you will use it most of the time because you basically offer drinks to friends or family members. "voulez-vous quelque chose à boire is more formal. You may never use it in your entire life unless you are working at a bar/ restaurant. The chance you would offer a drink to a stranger, your teacher, or a VIP is slim.