Just a few things off the top of my head:
- Don't use black to shade unless it's a black and white photo, black is for artists who don't know how to use color
- layers layers layers, layers are your best friend. Use them to organize, use them for shading/lighting modes, use them
- Use stabilizers for tricky line art, in Paintool Sai this is the top right option on the task bar
- Try not to overdo it with the blur tool, I see this mistake a lot with beginner artists
- Don't use super bright or saturated colors, I see this mistake a lot with beginners too specifically with digital art
- Experiment! There's a lot to discover with digital art and it's no risk at all to try something new!