Since the camera focuses at the beginning of the selfie timer cycle,
place a target at the position where you will be.
When the camera is focused on the target start the timer.
Move in to position and move the target out of the way.
The camera will be focused on you.
If you are using face detection, an 8x10 ‘headshot’ style print on a light stand or second ...
In my opinion:
learn the manual of your camera and how it works, the parameters you can play on and how they affect you pictures.
learn about yourself : how do you see the world, what do you like, what do you want to show ?
practice, wait a little, get back to your pictures, practice again...
I think the best way to start off is by exploring the environment you are in and try to find compositions.
You could use guidelines like rule of thirds, leading lines and learn about basic shots in photography like long shots, medium shots. Also focus on lighting of your subject.
You could actually begin with practising with your phone camera and later ...