Hi, i have a few questions
1) If i book now, can i cancel at any time?
Yes, you can cancel your booking at any time.
2) If i do decide to cancel my booking will there be a charge?
Depends on what you have booked and when you cancel.
If you have booked a PACKAGE reservation, you will be charged $100 if you cancel between 6 and 45 days prior to your arrival. If you cancel 5 or fewer days prior to arrival, you will be charged $200.
If you have booked a ROOM-ONLY reservation, there's no charge to cancel 6 or more days prior to arrival. If you cancel 5 or fewer days prior to arrival, you have to pay one night's stay, plus applicable taxes.
How do you know which you have booked? The easiest way is to look at the deposit that you paid. If you paid $200, you have a package reservation. If you paid ANYTHING else as a deposit, you booked a room-only reservation.
3) If a code comes out, how easy is it to apply it to my reservation?
Again, it depends on what you have booked! 
If you have a package and a code comes out for a package, you have to cancel the first reservation and make a new one with the code. The cancellation rules above for a package would apply.
If you have a room-only reservation, it's very easy to change. You just call and ask if there are any discounted room available for code XXX and ask them to apply that rate to your reservation. Very simple, and no charge to do it.
4) If i book my stay using the Disney World website, can i still phone up when a code comes out?
I don't believe so. I've heard conflicting reports, but I believe you would need to cancel your online reservation and rebook it with a code.
Thanks!