When filming, always keep the strongest light source behind you so that it illumines the scenery or the interviewee’s face. Some third-party apps like Filmic Pro and ProShot will allow you to use your camera’s flash as a light source, and on many phones the native camera will also allow you to do this, although with less control.
To fill shadows in dark areas of your photo or video, you can use an ordinary photography reflector to ‘bounce’ light from the primary light source into areas of the shot that are in shadow.
You may also choose to use an external light. Some tripod mounts have a cold-shoe mount to attach a portable smartphone light – and popular cold-shoe lights include selfie ring lights, and lights by Lume Cube and Manfrotto. Or you could try a photography soft-box light. These are usually lightweight and low-cost.
This tutorial from Primal Video is a great guide to using external lights:
This tutorial from CNet Australia is a couple years old now so the equipment recommendations aren’t up to date, but their advice on working with indoor and outdoor lighting is excellent if you’re new to video storytelling.
The BBC Academy has also published excellent tips on using external lights for TV journalism, and much of this advice applies equally to mobile journalism.