How far in advance does one check in?
As far as I know you can check in online a few weeks in advanced, and print off your boarding passes.
Is one assigned seats for free?
Ryanair operate a 'free seating' policy, you walk onto the plane and choose your seat like a bus (With the exception of row 1, 2, 16 and 17.
Can you choose your seat and if so does it cost more?
You can reserve any seats on row 1, 2, 16 and 17, think it costs £10 each way?
Where is the best place to sit assuming I am not desperate to get off the plane as soon as it touches down?
You have to pay for the best seats