Thank you! Depending on what you are going after. If it’s cell-shading, paint tool SAI is the better option. If it’s painting, photoshop would be easier. These two are the main programs that I use. There are people who do well with other programs such as painter and GIMP, etc.
If you are just starting out, SAI is probably the easier one to learn. I used to experiment with a ton of different programs before settling down, so don’t be afraid to try out different ones and see which one you are most comfortable with!