There's a very large selection on
http://www.holiday-rentals.co.uk.
Most owners (myself included) will answer any questions you may have then send you a booking form. When you return it they will ask for a reservation deposit with the balance payable eight weeks before you go. In addition you will be asked to pay a damage deposit which is returnable a couple of weeks after your return. I can honestly say that in four years I have never had to retain any part of the damage deposit since most guests leave the house in good order when they leave and no one is going to quibble about the odd breakage.
One word of advice: some owners are prepared to offer a one-bedroomed apartment to four people but two of you will end up on a sofa-bed in the living room. It's far more comfortable and not so much more expensive to rent a two-bedroomed apartment. Similarly, do make sure you let the owners know the total number in your party - I've heard of people being asked to leave for arriving with more people than they booked for...
It's a perfectly safe way to book accommodation, especially if you use one of the main sites such as the one above, since owners using these sites rely on them for their bookings and will not wish to jepodise future advertising. However, I recommend you make sure that anyone you are renting from uses a good local management company in case there are any problems during your stay.
In Santa Ponsa there are two distinct areas. The entertainment area, also known for historic reasons as the Irish Quarter, contains most of the bars, clubs and large hotels so is pretty lively. Ten minutes walk along the promenade, at the southern end of the bay, the area contains mostly residential apartment blocks and tends to be a tad quieter and has some of the better restaurants.
Have a great time!