Hi there.
First, it's a great sign that you're aware of your risk and that you want to find a form of birth control that works best for you when you decide to have penis-vagina sex. There are many methods available that don't rely on hormones, unlike those mentioned above. When choosing one, I generally counsel that you should try several methods to determine which is easiest to use, which you're most likely to use and which fits within the budget of you and your partner (he should definitely contribute to the cost).
Pills, patch, shot, ring: all these are hormonal. The barrier/chemical methods have high success rates when used properly. When you combine a barrier such as a condom or diaphragm with a spermicide and use properly, your risk is minimal.
I would encourage you to learn about all these methods. You can go to the excellent online site www.Scarleteen.com which has a great section on birth control options.
And you're correct: prior to ejaculating, men expel a fluid that can contain millions of sperm. For this reason, be sure you don't come in contact with a penis unless you're protected.
You mention that you have irregular periods. This must drive you crazy! Have you asked your gynecologist if there's another method besides hormones that might be an option for helping to regulate your cycle? Or you could ask the MedHelp Gynecology Expert at
http://www.medhelp.org/forums/show/103
Best of luck to you. Dr. J