Writing a biography speech is different from writing a corporate speech, as you'll want to tailor your style and facts to your audience. If you've been invited to give a biography speech (and if we can presume that the biography is your own), you can already assume that people are interested in your life. You'll want to be engaging, as well as factual, for you'll want to confirm for the audience at hand that your life has been worth giving a speech about, and therefore, that the organization you're speaking to was right to invite you.
Do some research about the organization that's asked you to speak, and tailor your talk to them. Are they conservative? Are they liberal? Are they for-profit, or non-profit? Do they skew older or younger? Choose the style you're going to write in accordingly, so as not to offend. Choose a theme for this speech that will dovetail with the organization's principles.
Choose the most interesting facts in your life to sum up your life and work. Ask yourself what your most interesting, as well as important memories are, with regards to your chosen theme. Jot these down, and try to clarify in your mind how much time you'll spend talking about each, given the allotted time that you have to speak.
Compose an outline, then when you're satisfied, write out your speech. Do keep the light parts light, and the dark parts engaging, as well as full of meaning for others. Think of the over-arching issues in your life, and make them as redeeming as possible. Don't hesitate to tell a few jokes to make your points, as well as tell engaging stories, but be truthful.
Perform your own timed rehearsal to ensure that you're able to keep to the time that's allotted, and do ask about the room where you'll be delivering the speech. Will there be someone to perform a sound check with you, if necessary? Request some bottled water to help smooth along your delivery. Finally, on the day of the speech, do some speech and throat-clearing exercises. When you give your speech, don't hesitate to make eye contact with members of the audience, it will enable you to be less nervous. Remember--your life is interesting! Let your audience know this to be true!