Check out the ten requirements to see if you qualify to register for the spelling bee at the Official National Spelling Bee website under the rules link. This is the first step in knowing that you will be able to participate in the competition.
Know the authorized resource for the words specified in the spelling bee is "Webster's Third New International Dictionary." Pick it up at your local retailer, second hand bookstore, or online. This is a major study source for all rounds except the second round. Other study resources can be found on the official spelling bee website.
Be prepared to take a written test. A sample of a test is given in the resources links on this page. The first round is a written 25-word test. All other rounds the words are given verbally.
Understand that pronunciation is helpful to sound out words, but it is not necessary to know how to say it correctly. All spellers are given a chance to pronounce the word before attempting to spell it. There is no penalty in the competition for pronouncing it incorrectly.
Spellers are expected to spell words within given time limits. Failure to do so will disqualify you from the spelling bee. Speed is important.
Remember all decisions of the competition are based solely upon the judges' discretion.