the basic tool I use for everything is Photoshop. You can have a 30day free trial version and then you’d have to pay. But there are alternatives for free which should be just as good for the beginning (just google “free painting software” or something).
The most important thing is, that you get yourself some sort of digital pen and tablet. They are available for roughly 60 to 100$ (wacom intuos S for example) and it’s well worth the price. You won’t get anywhere with a mouse if you want to get into digital painting.
I hope that helped,