This question comes up a lot in the 1Mby1M community, and I usually advise people to follow these specific steps:
1) Read lots of Entrepreneur Journeys case studies and get into the mindset of other successful entrepreneurs, what they have done, how they have done it.
2) Listen to lots of pitches and discussions around those pitches at the 1M1MRoundtables. This will give you a basis for understanding how ideas are evaluated.
In both exercises, jot down which ideas appeal to you and why. The point of this exercise is to find what you are passionate about.
3) Let the ideas simmer, and see what bubbles up as something you would like to work on. May be, 2-3 different ideas bubble up to the top.
4) Run the ideas through the 1M/1M Self-Assessment. See which one performs the best in this scrutiny.
This is probably your starting point to get going with something.
5) If you like, comes discuss with me at a Free 1M/1M Public Roundtable.
None of the above steps cost any money. All the resources are 100% free.
I wish you all the best.