There's no short answer but you can try this:
- Convince your team to believe in metrics.
- Choose the metrics that best work for your project -simplest one is number of completed user stories .
- Make everybody look at the same set of metrics.
- Put all metrics in just one place (a taskboard and burndown charts are more than probably you'll need).
- Create the habit for all your team to provide initial estimates and then compare them to their actual work.
- Do not waste too many cycles trying to innovate in metrics, everything has been already discovered, is just matter or look what would be good for your project.
- Metrics are not black and white, there's a lot of gray areas in them.
- Metrics are indicators, symptoms but not conclusions or explanations.