Practice makes perfect, there are tons of tools out there (free and paid) on the web that can help you improve!
As mentioned by people above, it all depends on what you are trying to paint that will help you find the best resource to teach you. Youtube anything and everything you've ever wanted to paint!
Other than that, there are some crazy painters out there on twitch.tv streaming how they paint like kaypike- http://www.twitch.tv/KayPikefashion
Another great resource is FABAtv.com- they have 5 new classes every month and have face and body painters from around the world come and teach their techniques. If you check them out on facebook, everyone posts their before and after pictures of their skills before they started learning on the channel, and it is really impressive!