Giving a presentation or teaching takes preparation. There are lots of tips and help for both, including the two links below in the "related section".
Lifehacker has a great article "How to Prep for a Presentation" with some great tips like knowing your audience, knowing their needs, knowing the subject, knowing the room (size, resources and technology, etc), rehearsing, getting rid of the jitters and nervousness and more.
There are some great tips for conference presenters and teachers there.
Survival Tips for Educators and Presenters
Backup plans - some tips for teachers