Going to Hawaii and need advice

Hey Hawaii experts, I need your help. I'm flying to Honolulu in June with three other friends.

For those of you who've been there:

Any advice about the best place for 4 people to stay on a studentish budget? Hotels? Hostels?

Is Oahu/Waikiki beach the place to be? Should we go see other islands?

What things to do are in your top five?