You need to decide what kind of digital art you want to create and which system you're using, this can guide your choice. For example the best drawing app for PC users can be Photoshop, and this does everything you'll need. But it's quite broad and deep, and so daunting.
If you want to paint art digitally then you'll want to go for software that simulates paint and how it blends; apps such as Rebelle, Corel Painter and ArtRage are fantastic for this.
There are more specialist apps too, for example Clip Paint Studio is ideal for comic artists and anyone who wants to replicate the manga style; this app has templates and guides to help and you can plan panels and even create comics in the software.
Most of the best digital art software will offer free trial versions to test out, and it's worth deciding if you want to pay an upfront cost or sign up to a monthly fee.