As far as how to proceed, you've got at least 6 months before most local seasons start anyway. Between now and then, build as much lean mass as possible. Then, about 12 weeks out, either hire a coach or at the very least find someone who has competed before and have them help you out. And as far as embarrassing yourself is concerned, I can promise that as long as your conditioning is halfway decent, you'll be far from the worst looking guy on stage. Also, competing is fun as hell and gets you shredded up for the summer, so even if you don't place it's still worth it.
Some federations ( list here ) such as the NPC or a show promoter (the person running the show) will allow you to cross over and compete in different classes within a show. For example, you may be able to compete in figure and bikini divisions or figure and physique in the same show. Other shows or federations do not allow that. You must check all the rules for that federation and specific show before registering. Most federation sites will have a list of shows for the year with the show promoters email so you can contact them ahead of time if need be for planning purposes.