Depending on destination, check out the tours offered by the big bus tour companies (cosmos, contiki, etc) or private operators. They usually hit the big sights in a reasonably sensible order and can give you some guidance. Just remember they make lots of quick stops that are planned and timed by the hour that you probably can't replicate exactly on your own, not should you want to if you want to really see and experience a place. Most guidebooks (lonely planet, rough guides, frommers, etc) will have a couple rough itineraries laid out as well. It depends on lots of things - rental car vs train vs bus, expensive vs budget, resorts vs hostels, cities vs countryside, food vs museums....

I find the planning almost as enjoyable as the travel itself. I've got enough rough ideas for trips planned to last me 20 years, now to find the time to take them. It will depend on your comfort and experience. I usually read a ton and have lots of options ready but actually plan little and take it as it comes and leave it flexible. You can get a lot of info from your fellow travellers on the ground that have just spent a month in your next stop.

You can also post a few details here or in the appropriate destination forum and get some ideas. Easier to help when your age, interests, destinations, budget, likes, dislikes are made known