I knew I was going to win the contest.
DId you ever think you might not win the contest?
No. I have the skills to win. It was over before it started.
It is good to be confident, but don't get overconfident. People will get annoyed by you and think you are cocky.
They should know how great I am.
You're right that it is fine to show off a little bit when you're good at something, but you may hurt the others who are just there to have a good time and compete.