Choosing a Medium
Anyone who has gone shopping for art supplies will know that there are a wide range of choices and a lot of intimidating artsy technical terms. As a young artist with no art training, I didn't know which supplies would give me the most for my hard-earned babysitting money, what I did and didn't need, and what I should buy first if I couldn't afford to get everything at once.
People often ask me what to get if they are learning to paint on a budget, or simply don't want to commit too much money to a new hobby at the beginning. This is the advice I usually give, based on my own trial and error experience. I hope it will be useful if you are starting out.
Choosing a Medium: Watercolor vs. Acrylic
There are several different kinds of paints, and each will produce a different look. I have experience with the two most popular, watercolor and acrylic, so I will discuss only these two.
Many people start already knowing which medium they want to work in, because they like the look of other paintings in that medium. This should be your foremost consideration, but there are many practical considerations as well. I started in watercolor because it was the only kind of paint that was available in the small town where I lived. I switched to acrylic about five years later. Watercolor and acrylic are by no means the only kinds of paint you can use, but they are the most popular and the ones most artists end up choosing between when they are learning. If you can, paint with another artist that will let you try their supplies. This might help you decide if you like a medium. If you can't, consider these things before buying:
Watercolor
The main practical advantage of watercolor is that the supplies are cheap and very portable. Your work is done on paper, which is good since you'll go through a lot when you're learning. You can bring a small watercolor palette and sketch book with you just about anywhere. Watercolor is easier to clean up, and will mostly wash out of your clothes if you get some on you. You may find that it is easier to get in the habit of regular painting practice if you can just pull out your sketchbook and paint something every day without much setup. If you're thrifty, you can get a decent set of paints, a brush and some watercolor paper for $30 or less. The main disadvantage: most artists will agree that watercolor is harder. There is no erasing or painting over mistakes, and paintings require more planning at the start. You'll also have to frame your paintings, which will add cost down the road.
Acrylic
In my opinion, acrylic paints are easier to learn with. Unlike with watercolor, you can paint over top of your mistakes, and even paint over the whole canvas and start again if you want to. When you paint on canvas, you don't need a frame, glass or matting - you can hang your painting right on the wall when you're finished. The main disadvantages: the supplies are less portable, harder to clean up and a little more expensive. You could pull out your sketch book and paint a quick watercolor sketch while waiting for the bus, but acrylic paint requires more space and setup time.
Think About Your Subject
Aside from the practical considerations of working with the medium, think about what you want to paint. If you know you want to work in large sizes (16" x 20" and larger) acrylic is the way to go, because it's harder to get watercolor paper in large sizes. If you want to bring your paints camping and paint the scenery, you won't have to bring as much stuff if you paint watercolor.
Subject matter with a low tolerance for error (portraits, realistic still-life) will be far easier in acrylic because you can make corrections easily. However, watercolor can add a degree of spontaneity to your paintings because it is more difficult to control.
A Word About Oil Paints
Many people starting out want to paint in oil, because they want to try this very famous and authentic medium just like the old masters did. I know some artists will disagree with me here, but having tried it myself I strongly recommend against oil, at least for beginners.
The difference between oil paints and most other paints is that oil paints are not water-soluble: you can't wash your brushes or dilute your paint with water. This means you will have to work with smelly, toxic solvents that require a lot of ventilation to work with safely, are bad for the environment and bad for your health. Oil paintings take days to dry instead of hours, which makes it much more difficult to paint over mistakes or do layering techniques. The paintings are difficult to move or transport when they are wet, so you have to have a safe place to put them where they won't be disturbed by children, pets or insects.
Many beginners aren't aware that acrylic paints contain exactly the same pigments as oil paints. Acrylic paints got a bad rap in the 1950s when they were new and the colors weren't very good, but modern acrylics are excellent. Most people (myself included, I have to admit) aren't often able to tell oil and acrylic paintings apart.
My recommendation: if you value the experience of working in oil from a historical perspective, try oil paints once you've had some practice painting and time to develop your style. As a beginner, you can learn most of the same techniques in acrylic without the frustration of working with solvents.