@ laila
two questions that you must be well prepared of are
1.Research Area/Research Proposal/Rationale of your research/Benefits to host and home country
2. why this country for PhD.(To impress them you must know basic information about country of your choice gdp,population, strengths, major exports etc/university in which you want to pursue phd.When was it established.ranking,Number of schools and departments,campuses,research facilities,supervisors,their areas of interest etc)