If you are staying at a Disney resort, you can make ressies at least 30 days ahead of time. When I was there last October, the paperwork I received I think said that ressies could be made 60 days ahead though.
If you have a good idea when you want to eat there and you are staying on property, I'd do it as soon as possible. I'm not positive, but I think the sooner you make your ressies, the better your seating will be, and the faster you will be seated, esp. if you have the first lunch ressie. We got there 15 minutes before our time and it was a madhouse! We weren't able to sit very close to the water either. Still a beautiful restaurant though! Check one of the posts on the top about PSs for the phone number to call. If you decide to make your ressie when you get there (like I did), there's a button on the phone in the room for Disney Dining.
If you are NOT staying at a Disney resort, you can go straight to the restaurant when the park opens in the morning and someone should be standing by the podium there. I've also read that you can make ressies at City Hall, but haven't done it myself.
Again, there should be a thread close to the top about PS's/Disney Dining that you might find helpful.