1. Requirements. Make sure that your customer defines their requirements in depth. You need to know exactly what must be delivered. Be specific, write them formally, and get them approved. This document will become one of the baselines upon which to measure your success.There's far more to it than this. Your client/customer will have a strategic object for its project. It might not be clearly stated or even understood, but every project has to advance the organisation's mission, or it is squandering its resources.
So, first: how does this project advance the mission? What strategic role will it fill?
Then, requirements. But 'requirements' is not just a list of things. To fulfil its objective a project must produce a level of performance, and have trade-off rules because you will never have enough money to do everything you want.
When you are refining the requirements and trade-offs, also define acceptance criteria for the requirements. Be clear hear because these will drive cost or performance. Acceptance criteria too high: performance required too restrictive, then costs could sky rocket. Performance set too broadly and performance might not be adequate. Either way you have to know when you're done and what 'done' looks like (to borrow a phrase that Glen Alleman uses).
Get those set first, then you can talk about 'requirements' and start bringing resources to bear.
But all this needs to be done with people: the people who will use the project's output: sponsor/s, customer/s, users and other 'stakeholders' or interested groups (regulators?).