We try to book our cabin at least a year in advance. Overseas flights can wait til 6 months out. Flights to North American embarkations can wait til 3 months. Once the cabin is booked we research ports of call for excursion possibilities, booking as we find them.
If you are brand new to cruising, and going solo, I'd really recommend finding a cruise specialist TA who can help you understand the solo supplement charged by most lines, or can steer you to ships with solo cabins. They can also help you find a cruise within your budget with an itinerary you want.