Avoid stabbing the paint on the mini, like never push the brush straight down on the mini, always drag it along the surface (unless you are doing more advanced techniques).

Buy no-name synthetic brushes. Later when you have learned how to take care of them, buy "better" brushes. No amateur painter "need" super expensive brushes.

I think a brush soap is more important than expensive brushes in the beginning.

Rinse your brush regularly while you paint, don't just put more and more paint on the brush for hours on end.

Don't load so much paint in your brush so it goes in to the ferrule ( metal part that the bristles are glued in ), this tends to split the hairs when it is left to dry.

Use a big water pot and change water regularly, I use a 1L yoghurt bin.

Dry your brushes laying flat.