I've heard it takes 3,000 hours to master a craft, whether it's drawing, creating music or whatever. So, usually we tend to master the things we love because we enjoy the 3,000 hours of practice it takes to get good at them.
A famous musician was once asked "How do I get to Carnige Hall?"
His response was always the same: "Three things: practice, practice and practice."
The hardest thing sometimes is looking at what you can do, comparing it to the output of these people who HAVE mastered the craft and then NOT saying "This really sucks, there's no need to keep doing it."
Right now, I've just started doing a tileset, and I'm sure it won't be anywhere near Sharm or Celianna's amazing skill levels, but I'm sure they've been doing that for years to get their skill up to that level. And I'm sure before they did tilesets, they enjoyed doing lots of drawing/painting/sketching, which gave them excellent eyes for color, shadowing, etc.
I think finding really good "How to" books or tutorial videos is a great starting point. Then just practice. On the bright side, there are a ton of free and very inexpensive tools you can use, and even professional artists use them. MsLittlefish uses LMMS (free DAW software), for example, and Sharm likes Pyxel Edit (which I think is around $10) for tilesets, but you can certainly use GIMP which is free.