Photoshop, you can get it for free, just follow a YT tutorial on how to get it. Pull the texture templates from google, they help allot, use a brush that mimics the movement of a real paintbrush, my favourite brush is labelled: 'round angle low stiffness'. If you want to make shading, choose a colour darker than the base colour, turn the opacity of the brush down to around 20-2% and slowly build up the darker tones where you want them by repeatedly going over the areas you want to be darker. Also keep in mind that it takes ages to get good, Iv'e been doing graphic design for a few years now, and art all my life, when i was a kid i would get angry because i couldn't draw what i could picture in my head, my mum told me "you don't become an artist overnight" and its stuck with me, the hardest part is to keep going, and don't be picky about how your art turns out
These are perfect examples of the technique i just talked about, i made these art pieces using the brush method i talked about, the method works with colour also.
GL!