How to use single journey subway tickets in hong kong?

Please give me some advise , i going to avenue of star and then wanna going to " clock tower " . I've read many suggestion with using MTR by single journey tickets . But they said that tsim sha tsui with EAST tsim sha tsui is separate so must buy two different ticket ?? I still not understand . I just want from avunue to " clock tower " . I hope you can help me / gve me good ways for this . Thanks before