Hello,
Brushing technique is very important to keep the teeth healthy.
Brushing at night just before sleep is important and as you said your teeth are cavity prones then do visit a dentist in every 6months.
With retainers ,you should clean your teeth after every meal.
Next Steps
Follow proper brushing technique,you can learn from YouTube too.
Health Tips
With ortho braces and retainers,clean your teeth after every meal!