Brushes Guide
Finding the right brush is important. Thankfully brushes are not expensive and you can buy a good one for around $5.
If you are only wanting one brush, I recommend a size 2 round brush.
This brush is fine enough to paint smaller areas and get good precision.
If you are wanting a second one, a size 4 is good to have for large areas to paint.
When buying paint palette sets, they often come with a brush so I recommend buying the paint first.
This brush from Officeworks is one I use regularly and it works really well for this type of colouring in painting.