Just go to SpanAir or Iberia and by a ticket.
www.spanair.com or
www.iberia.com. Or you take the speed ferry across. I would suggest not buying your ticket till you get there because you can get a better deal at the airport or from an agent in Spain. You're getting there about a month before the opening parties they start happening in June. That shouldn't put you off of going though. Just being on the island is really special. I'll be there in September. Try to stay in Ibiza Town. If possible stay away from San Antonio (The West End). Oh, and its also really easy to miss your flight off the island. You just keep putting off getting to the airport. And Barcelona is the shit too. There is plenty for you to do there. To bad your not there for the Sonar Music Festival in June. Good Luck!