Please note that I am not an expert in painting, but rather speak from my own experience and training.
I've already left some tips for beginners in Surfaces, Paint, and Brushes. There will be a tiny overlap in reminding you to experiment. If you're new to painting, that's fine and I'm excited for your journey, but I am not here to explain the basics of colour mixing, how to compose a painting, or how to paint. There are plenty of classes available either at your local community centre or college, or through private instruction. Trust me, they will be much better instructors than I. Google will be your friend in finding classes geared towards colour theory, how to paint, and what makes a great composition. There are also countless YouTube tutorials on the topic, and those painters are really quite proficient in explaining their techniques.
Don't be precious
The hardest thing when you're learning about painting is fear. Fear that you'll make a mistake, wreck a painting that you're happy with the progress on, mess up your expensive canvas, or waste paint as you're navigate through colour mixing. Try to avoid seeing preciousness in everything you do. When you make mistakes, you'll learn from them. You'll learn what works, what doesn't, and why that is. You'll see the next piece with more experienced eyes and a better grasp on how to control the paint.
Don't feel constrained by the rules of painting with oils or acrylics (or even watercolours and gouache). Instead, experiment with paint application, tools, colour pairing, composition, and even mediums that are frowned upon. During my first time in art school, I was told that metallics and fluorescents were garish and should be never be used under any circumstances. For years, I avoided them. Now I use them liberally and without fear. Don't let arbitrary rules you'll find in certain art forums that are based more on personal taste stop you from finding your own way.
Paint like you
I often get asked about developing your own style. That happens naturally. Instead of worrying about painting like someone you admire, focus on painting like YOU and your style will develop naturally. The work I admire and love is often much different in style and technique than my own. I love hyperrealism but it's never something I would pursue as an artist, even though I deeply admire the artists who accomplish that.
Do a reproduction
One of the ways I learned colour theory was using the books Color Mixing Recipes for Oils and Acrylics and Color Mixing Recipes for Portraits to create a reproduction of a painting in art school. To do this, you choose a painting that is considered part of the public domain (to avoid copyright infringement, so think dead masters like the early impressionists) and try to create a reproduction with both colour and brushstroke. This is how you begin to understand what pigments will create what colours when mixed together. It's a wonderful challenge and worth attempting when you're learning. I have to stress, do not reproduce a living artist's work without explicit permission. You have a high chance of copyright infringement, and no one wants a lawsuit on their hands.
If you have other questions, please leave them in the comments below.