First decide your area, i.e. in fields of CS, which one you want to go for. Then further narrow down your scope based on your interest in particular topic of that area. Furthermore you can see the faculty in your univ working on your desired area, and decide exact topic by their consultation.
Furthermore you can google out for ideas but you have to be specific.

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I am not interested in power for power's sake, but I'm interested in power that is moral, that is right and that is good.