I think everyone who does the RTW trips have their own way of planning but this was what I did.
First of all you need to know your budget and how you will get the money together.
Things to consider which need to be included are the costs for
Transport, round the world tickets plus add ons if needed or buy as you go.
Will you be doing major train journey, fro example in Japan you should by the tickets in your own country as this works out cheaper.
If you intend travelling by coach just check out the coach companies in the countries you intend visiting.
If you intend camping, you could rent a camper van, we did for Australia.
Cost in for Malaria tablets, these are very expensive and any inoculations or injections you need, always speak to a healthcare professional with regard to this.
If you are on medication talk to your GP about your trip.
Passport make sure you have at least 6 months on it and enough pages, pages can be added, ask the passport office.
Visas another big expense and you need to check out the countries requirements, some you will be able to buy in advance some will start from date of issue, some you will need to buy while you are travelling. Check out which countries you need to purchase on route and list the cities where you can buy them and what their requirements are. Take a number of passport photographs with you for the Visas.
I would always recommend taking more than one debit and credit card. Cards stop working for no apparent reason. Check out the banks that will give you the best deal. Never keep all your cards in the same place, BIG MISTAKE. Purchase a money belt. Have all your contact details available if you loose anything.
If you use Internet banking DON'T use Internet cafes, the security may not be reliable. Use 5* hotel business offices, it may cost you but the line will be safe. Drip feed money into your accounts by using the standing order system, don't put all the money you have in.
You need to make a list of all the countries you would like to visit including the cities trying to estimate the number of days/weeks/months you wish to stay there.
I set up a spreadsheet of all our days 12 months in all and allocated days for each place, this gives a rough guide as to the amount of time you have and if it's possible to fit in all the countries.
Some countries/cities you will enjoy visiting more than others so you might add on days some you'll not want to stay as long, you need to be flexible and not have things written in stone after all this is a once in a lifetime trip and you want to take back great memories of your time not go back home thinking I wish more time in where ever.
You'll need to do masses of research to arrive at the countries/cities list but this is all part of the fun and should be enjoyable.
Ask questions on all the travellers websites you'll get loads of information from experienced travellers another website is
www.hospitalityclub is a good site for meeting local people, check it out. We met and stayed with many locals on our trip and I can recommend it but do have an e-mail relationship with the people you might be meeting up with if this appeals.
Oh and don't forget be careful out there not everyone has your best interests at heart.
Best of Luck