No need to book for a tour as you can go here on your own. Head down to Myeongdong station and find the exit ( look for the Pacific Hotel) to the street leading to the tower.
There is a cable car going up to the tower ( 8,500 won for a return ticket ). Entrance to the observatory is not included. I suggest to get the one way trip ( going up ) and walk down on your own as there will be lots of photo-op here. It's not that high.
The view is amazing on top, as you can see below.
Up above the mountain, Alive Museum, Locks of Love ( see photos below ) and there are a lot restaurants as well.
Other info, please go here.
The street leading up to the N Seoul Tower. There's a lot of cute shops here. You'll be entertained while walking up that hill.