Do I have to buy ticket or stuff to go and watch?
It depends on the contest - for Worlds in the US, yes. Most US regionals, no.
How much is it? And when?
Google “worlds yoyo contest” and “National yoyo contest”
To attend, most contests are free. I know Worlds charges and the charges will be on their site about 6 weeks or more before the event.
I know CalStates, BAC and Nationals are free.
Just about all contests have entry fees to compete.