Hello,
All you need for the interview is to be fully aware of what you've written in your application form. Your school and shell UK have already reviewed your application before you were nominated and called for the interview in the first place. You therefore need to proof to them that you are all that was written (and probably more) but I'll advise you don't go too far from your application contents to avoid contradictions.
Moreover, you must be confident and convincing enough in stating how your intended course would aid the sustainable development of Nigeria on your return plus your personal and career plans.
To be frank, it's more of a discussion than a rigorous interview at this stage because your application has gone through some rigorous processes before you were called.
All said, the ultimate, as far as i'm concerned is God's favour. Therefore, pray also.
I wish you the very best.
.........To God Be The Glory