I was the same as you except I always wanted to travel on a 747. On a recent trip to Hong Kong I checked in online waaaay back in row 47 so that I could appreciate walking the length of the big bird and get a feel for the size.
As it happened I ended up in Club on the upper deck. It's like your own private jet up there, sure you'll enjoy it.
You can check-in online up to 24 hours in advance. If you don't leave it to the last minute you should be able to grab some upper deck seats. Check out that seatguru website mentioned above too. Some useful info.