I've been given a Fitbit type tracker - now what?

[AsAProfessionalFekko]

Lots of flashing lights and targets.

How do you use them sensible (so that I'm not constantly looking at the app and wondering where those extra 32 calories went?)

You can probably use the app to set targets that are meaningful. I use 10,000 steps and 60 active minutes a day.

Active minutes per day is the best target, 10,000 steps goal has been debunked. It depends whether you enjoy tracking yourself or not. I use mine to keep me in check if I've been a lazy bum, and the heartrate stuff is helpful to know if I'm trying my hardest at the gym.