Reguarding which brushes to use, this is a highly sugestive questions depending on what you create, your style and how you work. Krita also has a pretty comprehensive brush editer (the hotkey for this menu is f5) The manual has a section dedicated to the brush editer which can be read here.
I would start with the brushes that are already included with the program first. Any of the brushes can be copied and edited to how you want with the amount of settings that are assigned to each brush engine. If the included brushes are not to your liking and would like to use what others have created, you can import or create your own bundle of brushes (which can then be shared with other Krita users). Some of the most popular brush bundles on the fourms are linked below.
If you have not added/imported any reasources (brushes for example) before, I highly recommend reading the manual for importing reasources here. Some reasources require Krita to be closed and then opened before they appear; also not all brushes have their own tags/catagories so some brushes may be in the “All untagged” catagory.
Pencil Sketches Bundle by Ramon Miranda
Version 1 Brush Set by Rakurri
Version 2 Brush Set by Rakurri
Foliage Brush Bundle by CrazyCatBird/Gymnart
MyPaint Brush Bundle by Fizzyflower
Essential Brushset Version 2 by Fizzyflower
Won’t Teach You To Draw Brushpack by wojtryb