Firstly, lighting is key – find a window with natural light, invest in a selfie light or shoot in the early morning or ‘golden hour’ before sunset. Make sure you keep the camera above you, so you don’t suffer from the double chin effect.
Smile with your eyes, and try to relax – you don’t want to look unnatural or forced.
Use the right filters, like using Adobe Lightroom (free on mobile), VSCO Cam or SnapSeed. And keep trying – repeat, repeat, repeat.