Voulez-vous boire quelque chose?
I would like to know how to ask a friend if they'd like something to drink.
I think the right phrase is:
Voulez-vous boire quelque chose?

Question

Voulez-vous boire quelque chose?
I would like to know how to ask a friend if they'd like something to drink.  
I think the right phrase is:
Voulez-vous boire quelque chose?

Answer

It is correct ,but if he is a close friend you can use ' veux tu ' , because 'voulez vous' is a formal polite way .
Tu veux boire quelque chose ?

or 
Veux-tu quelque chose à boire ?  
Would you like something to drink ?

J'espère que c'est clair :)

Answer

Your answer is correct, though it would be "formal" or plural form.
For '"informal" or singular "Veux-tu boire quelque chose?" will do.