Write tweets that people would want to read. Tweet about relevant topics and events (although I'd stay away from politics as that's controversial and could lose you followers), tweet about stuff that's trending, or anything that you're interested in, or even just things that you find funny. Reply to other people's tweets so that people see your account, and use hashtags so your tweets will come up in searches. If you want a celebrity to follow you back, reply to their tweets, retweet their tweets and make sure you're following them. You could also tweet about them from time to time to get their attention.