Take Trial Ad I at your school.
Alternately, the book, Basic Trial Advocacy, by Peter Murray is decent.
Few opening statement tips:
1. avoid argument
2. use descriptive facts [Not "it was a cold night and a long walk for Louie", but "His cheeks were chapped by the time Louie had walked the four blocks back to his apartment, fortunately his wife had hot tea at the table for him..."
3. simple visual aids, if relevant
4. tell a story [beginning, middle, end] - like you care about it
5. weave in some personal anecdote about you, if it adds to your credibility and people would find it interesting [open statement is first opportunity to sell the factfinder on your credibility]