Are you a student or a staff member? The topic you choose depends on the theme of the conference. Usually, the conference organizers will have mentioned it in their publicity material. When and where will the conference be? You should choose a topic from the given theme that you feel passionate about, as if you do not, then your performance will be substandard.
I have given presentations at PESIT in the past.
Have a nice day.
Regards, Simhan