I think where I would start is by gauging local interest. You will need to consider venues, perhaps before this, so you can give your players a suggestion of cost. You will also need to consider if you want the event catered or send your players out for food, because this will affect the cost. You should ask your players what size armies they have, because this will help you determine length of time for your games. You will want a venue with tables and chairs, make sure you check for this, or you might be unpleasantly surprised the day of!
You can also bother gaming companies for prizes. Although GW may not sponsor you, paint, brush and scenics companies might. Do you want to have a painting competition at your tournament? Will you have someone running demos for the public? Will you have a "gamer's garage sale" table?As an organizer, you should be prepared to have a fellow organizer or even yourself if you are so prepared to play a game if someone doesn't show and you have odd numbers. Of course, you can just give a buy, but that's not as much fun for the player. (Been there.) Who is making/bringing/buying terrain?
Will your tourney be Swiss style or Round Robin? Both have their advantages; Swiss is quicker, but Robin lets everyone play everyone. I would probably run Swiss as a prize/weekend tourney and Robin as a FLGS league. Other game companies and stores do this and it works well. If you are a store, you can do your RR with a chance for players to improve their armies between matches by purchasing new models.
You can also ask on other website forums or Facebook groups for help. Write the editors of gaming magazines like Wargames Illustrated for suggestions or sponsorship. I read an article on organizing tournies in just that mag.
Hope this helps!
Come to think of it, this is a lot like planning a wedding. Except you don't have to get a liquor license (or do you?
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