1) Find out about your audience. If you're engaged to provide a particular service, whether or not you'll be paid, then you'll be more effective if you know a bit about the people you'll be talking to. Consider things like the age, gender, professional background, life experience, what they may have in common as a group, and what they will know about the subject of your talk. Learning things like when and where you'll give your talk also offer insight into the possible frame of mind of your audience. Will they be in a serious mood or 'laid back?' Is there a chance they'll be drunk? All of this will help you choose a talk that will have more widespread appeal.
2) Assess the purpose. Are you planning to entertain? to inform? to persuade? elicit information? motivate donation-giving? What does the topic mean to your audience? They will have more respect for you as a speaker if you're confident about what you're doing and they relate to your talk. If you come up with appropriate analogies that enable audience to feel you dialogue with them, then the tone, word choices, and subject matter will coincide and strengthen your talk. Audience may interject.
3) Prepare your material. Some people have a knack for speaking intelligently 'off the cuff.' Yet, the majority of good speakers research their topic, practice the talk, get input from other people about material or jokes, and take other steps to generally prepare in advance. The more you know about the speech situation, mood of audience (happy or sad), and nature of the venue, the better prepared you'll be.