Having worked quite a few charities in the UK, I would definitely advise you to get involved with a local charity to familiarize yourself with volunteering work.
My second advice would be to register on websites such as idealist.org, devex.com and LinkedIn. There are hundreds of offers for volunteering all around the world and you can see the type of experience and skills they require.
It takes about 9 month to find a volunteering position from the day you decide that's what you want to do and the day you actually leave.
VSO is a British organization and you'll probably need to be in London to go through their selection process (I did it over 4 years ago in London and I have to say that it was quite thorough).
Finally, you can check international charities and see if they are looking for volunteers for their overseas actions.
In term of preparation, all the larger charities should ask you to attend their orientation program to get to know their work, how they operate and the extend of their activities overseas. I participated in a preparation weekend for a French charity and it was really useful.
Finally, you can go to a specific country and look for a volunteering position there. Depending on your skills, they might be very happy that you contacted them directly.