First and foremost it's important to be confident in your subject. It's OK, normal even, not to know absolutely everything when you are training, but you need to come across as confident if you want an attentive audience. People associate confidence with competence. Read around, learn as much as you can, but don't be afraid to tell someone that you will get back to them later if you don't know the answer immediately. Keep a pen and notebook with you to write down questions that you don't know the answer to and make a point of learning the answers before the next group session.
Secondly, practice in front of a mirror. Weird body movements and nervous gestures can be very off-putting for an audience. Standard presentation advice includes standing still, keeping your hands free and relaxed, and not doing things that might distract a training group. If you find it very hard to look natural when presenting to a group, ways around the problem might include structuring your talk with PowerPoint or arranging for all of you to sit around a table to create the less formal feeling of a seminar. If you are particularly nervous, enlist the help of friends and family and practice at home until you are comfortable.
Don't get bogged down in the text
Use keywords or picture diagrams for your notes. Don't write down every word you want to say as this will just distract you. Your job as a trainer is to observe and listen to your group as much as it is for them to listen to you, and for this you will need to be aware of the group dynamic and body language. Reading text from a page will keep your attention focused squarely on the piece of paper, raising the risk that you will ignore your audience.
Finally, make presentation skills a part of your on-going personal learning experience. There are many websites and learning tools that offer advice and training on everything from body language, eye contact, diction and pace, to more advanced stills such as presenting to a multicultural audience. People who make the best trainers see the learning process as being continual, so taking a reflective approach to every presenting experience is an invaluable habit to adopt.
This is a big subject, but hopefully this basic advice will help. There are other trainers here; what are your top tips?