Ticket price indeed varies greatly depending on starting location. You must also end there, with some allowed exceptions, so total costs might include getting to and from starting place to home.

There's an Excel spreadsheet referenced in one of the stickies that gives prices for every possible OWE trip from every possible starting point. In Europe Romania and Bulgaria may look cheap but the fares listed in USD are being charged in EUR by BA, the only choice there. There's been chat about Finland being cheap, but that may not be for AONE3 - "cheapest country" varies by class and number of continents.

It's best to book directly with one of the OW airlines - few travel agents are knowledgeable enough to make anything but a disaster of the job. Note, since it's a sensitive topic, that I didn't say "no TA's". I've booked with them, but only with a name that was recommended by a correspondent I trusted.

With AA and BA you ask specifically for the "round the world desk". Their direct numbers are in the sticky somewhere, but any agent will transfer you.
Other airlines may not have dedicated people, in which case the agent you're speaking with will offer to help you. It's best to have become so wise by browsing here that you have no questions for them, merely an itinerary that you know is valid for them to book and price.

Which airline? Usually the one you like dealing with the best, except that some won't do the booking if they don't have the first long leg. They're polite when they say 'no'.

For route planning, visit CathayPacific.com and download their Trip Planner (for all OW airlines). It's not so flexible but it's about all we've got.

You probably know that you'll only be able to book flights with seats in the appropriate discount category - A, D, L - although you can bump yourself down to any available lower cabin if you really want a ride badly. Unfortunately, A doesn't bump to J - you still need a D seat even if you deserve a ride in the front. In most cases availability is pretty good. A service at ExpertFlyer.com at $10/month is a good way to see actual availability, when you get close to finalizing.

You probably know by now that if you hate the itinerary after you've done some of it, you can revise it for $125 or so.

Note that you have a year to do the trip. You can spend arbitrarily long anywhere you please. And as mentioned regarding South Pacific, you're free to buy and use separate tickets to connect points on your OWE or to go home from halfway, and return later to continue.

Take a friend - it's only money.

Speaking of which, they'll get about USD1000 out of you for "fuel surcharges" and "taxes", above the listed prices.