Hey - don't know if you'll check this but here is the advice I have for you.
I spent a very long time pondering what my need was going to be because I thought that I had to actually find a problem in the world and I thought that my product had to solve it. But, I found that the need doesn't have to be that broad. It can be a personal need. Eg - I needed a formal dress for my formal this year. I can't find one because of my taste and body measurements - BAM! MDP idea. Try not to think too big because you are just making more work for yourself.
Secondly, coming up with an original idea is very hard. Therefore, innovation is key. Once you have an idea, go and investigate what you can already buy. Find elements of each thing and adapt it to make your final design.
I hope this helps you. It's what really stuck in my head when discussing with my DT teacher.
Good luck!