There are many details that go into conducting a successful Solo and Ensemble contest. The best way to get started is to participate in an existing contest in your area, either as a Band Director with students, or perhaps as a performance Judge.
At a minimum, you should consider the size of the contest as appropriate for the number and size of available rooms you have. You should recruit judges from local band directors, music professors, and tutors, remembering that each judge will expect an honorarium payment, normally between $125 and $500 USD, depending upon the number of students you expect them to examine.
The simplest kind of contest is one with only students from your own school (or perhaps within your local school district). That gives you a chance to see how the contest is run, and is lower stress than starting with a larger contest that includes schools outside your local district.
Solo-Ensemble.com is considering hosting an online forum (discussion area) for Band Directors to share best practices and lessons learned from managing existing contests with Solo-Ensemble.com. Stay tuned to Solo-Ensemble.com for details.