- Research, research, research
- List your "top ten" (or 15!) destinations (not even taking into account this trip and logistics yet, just in general) You may already see that most of your locations are close to each other and would make for a good trip or you might see that they are scattered all over the globe and you'll need to take more things into account.
- Decide why you are doing this trip. Is it to go to every continent? Get to know a specific region really well? Visit the wonders of the world? To literally go around the world? Once you have a true purpose and know what you want to get out of this adventure you can better formulate a vision and break it down into smaller goals.
- Look at your time frame and budget and start researching costs: flights, accommodations, food. Long haul flights across continents will add up fast. Certain regions of the world are just cheaper in general and your money will go farther. If visiting one of your "top" destinations is important to you because you want to do a certain activity or see a certain event, take those costs into account as well.
- Consider timing. If you are traveling from January to March and want to enjoy tropical weather, don't plan a Trans-Siberian trip from Russia to China!
- Talk to people. Again, more research. The more you learn, the easier it will be to figure out what you are OK with sacrificing, which brings me to my final point.
- Focus on your main goals and be OK with sacrificing some places to save for later. Let's face it...at some point you're going to have to say, I just can't do that on this trip. It doesn't mean you'll never go to that specific destination or that your trip won't be everything you've dreamed of and more.