We Love is a day & night affair. When you enter, you're given a wristband (a.k.a. a pass-out) that allows you to exit and return one time.
My advice is to go mid-afternoon (around 3) and spend some time on the terrace. Take a break in the evening (go to Bora Bora, get some food, drink a gallon of water, whatever) then head back in for the night program. All this contingent, of course, on which DJs are playing when.
Yes, you will need a ticket. If you buy them through Ibiza-Spotlight, they cost the same as at ticket outlets but you get VIP entrance (just take a photo ID with you) . When I went for the WLS opening, there was 1 person in front of me in the VIP line and about 40 people waiting in the main line.