You must convince people to retweet you or repeat a phrase in their tweets. If you create some buzz, Twitter algorithms will notice the trend and display it on the trend list. Famous people who have lots of followers (@ladygaga, @justinbieber etc.) can create trends with just one tweet because their audience is huge, so people talk ybout their tweet and they get many retweets.
The Twitter micro blogging website operates an algorithm to determine which topics are the most discussed via Twitter users at any given time.
According to me On the Home, Discover, and Search pages on the web. Trends appear in the column on the left side of the page.
It would be difficult to start a trend because of the need to get hundreds of people to use the same hashtag. I started a blog yesterday, it's a comical commentary on the current trending topics. http://hashtagwhatstrending.blogspot.co.uk