Hey Mona,
I've been to Santoini, Ios, Naxos and Paros a couple of times island hopping with friends you'll absolutely love it! For a group of girls your age Ios is and absolute must, you won't wana leave when you get there, much livlier than Santorini but not as much to do or see during the day, purely a party island with a couple of very nice beaches, I absolutely loved it!
Naxos is a much bigger island and caters for families mostly but still has plenty of pubs and a couple of niteclubs...really nice place to visit after being in Ios as it's very relaxed, lots of nice restaurants, creperies, ice-cream places and great for some water sports also! Paros is a mix of Naxos and Ios in terms of livliness, with all nightlife based in one area called the 24-hours quare(if I remember correctly!)
If I was going back and had to proritise I'd go 1st Ios, 2nd Santorini, 3rd Naxos and 4th Paros...great places!
I'd def wait until you get off the ferries and make a deal with people at the port for accom, you'll get it much cheaper there than if you booked online in advance. One place I'd def reccommend in Ios is called Rita's Place, really nice setting, staff and pool area.
Any more questions please ask! You'll have a savage time!
Ian