There’s a good reason that we say, “Practice makes perfect!” You simply cannot be a confident, compelling speaker without practice.
To get practice, seek opportunities to speak in front of others.
If you’re going to be delivering a presentation or prepared speech, create it as early as possible. The earlier you put it together, the more time you’ll have to practice.
Practice it plenty of times alone, using the resources you’ll rely on at the event, and, as you practice, tweak your words until they flow smoothly and easily.
Then, if appropriate, do a dummy run in front of a small audience: this will help you calm your jitters and make you feel more comfortable with the material. Your audience can also give you useful feedback, both on your material and on your performance.