What do I have to do in preparation for applying for a PhD? I'm just about to go into my final year of my computing degree, and I want to continue to a PhD after completion. What preparations do I need to do prior to applying for grants/scholarships/funding?
Get good grades. 
Make sure (some of) your lecturers know who you are, so that you can ask for a recommendation letter.
Get involved in some kind of research, either a thesis or something extracurricular.
Work out what topic you want to study; at least have a first idea.
Select relevant coursework based on the previous point.
Don't be afraid of going abroad. Prepare yourself mentally for this now, so when the time comes to actually decide, you'll be ready. Check details such as visa requirements, as these can take 3-4 months to obtain.
Research good universities who do what you want to do. Find out about their application procedure is and their deadline.