You know what work needs to be done, but don't know where to start?
That state is very common and not per se bad, because you've got a lot of opportunities within your complex system and decision important and by no means easy.
As said, there are a lot of ways to prioritize your backlog:
- highest risk first
- biggest expected value
- most urgency or deadline
- feature deprecation
- market opportunity
- caustic curve
These ones are rather good, but will they give you the hint were to start? Usually your experience will.
Personally I trust my gut very well and I am impatient about all the sensitives around biggest value and all discussion about unknown future. I like to gather feedback.
People who are shifting from classic to agile still got huge problems with starting, so I recommend: start with the work which is ready for development. Only work in ready state will get done without being blocked and is able to finish within a short period of time. So stick with this: "There is little point in paying attention to prioritization when there is no predictability in delivery. Why waste effort trying to order the input when there is no predictability in delivery?" (David J. Anderson, KANBAN, 2010, p. 31).
You should focus on improving your ability to deliver and the predictability in delivery - this starts with making work ready for development and not with doing estimates, preinvestigation. This is where you should start.