What are some good techniques to use for painting on a canvas?
Of course, there are nearly endless techniques to use when painting... in the world of art, you could potentially even invent your own technique! However, I can offer you some advice about starting out on canvas as a beginner, to give you a beginning point for exploring your own methods. Since it is the most commonly used type of paint for amateurs, I'm going to be referring to painting with acrylics (see the enotes Wikipedia link below for even more about acrylic painting techniques in addition to my answer).
Depending on the type of canvas you purchase, you may wish to apply gesso--a white priming medium--to the surface before actually starting your painting. Most pre-mounted canvases, such as those you can purchase at a craft store, are already primed and ready to go. If you mount your own canvas, though, applying gesso will prevent the paint from sinking into the fabric and will give you a solid base to start with.
Acrylic paints normally come in tubes of various sizes and various quality, depending on your budget and skill level. If you have an art supply store near you, the employees will give you excellent information on what to buy based on those parameters. They may also offer painting classes, which can help you expand your repertoire of artistic skills!
Start by laying down a base color that approximately matches the shade and hue of your dominant background. You will be able to layer new colors on top very easily. Acrylic is very opaque, so you can re-adjust almost endlessly. The process of painting with acrylics is based on the idea of applying, mixing, and blending colors to achieve your desired effect, with the knowledge that the paint dries quickly and permanently (unlike oil paints). Acrylic paint will blend beautifully for you, as long as you're working with fresh, wet paint. White and black (or a very dark grey, purple, or brown) are essential colors to have on your palette for creating shadows and highlights. This helps achieve a realistic looking, three-dimensional image. Of course, if you are painting abstractly, you might be focusing more on flat blocks of color. It's up to you!
I generally recommend purchasing a selection of three brushes to start your collection out--the exact size of the brushes depends on the size of your canvas and amount of detail you want to achieve, but select a relatively large brush for sweeping on big areas of color, a medium size one for outlining, filling, and blending, and a small one for detail work. Synthetic brushes are the least expensive, and work fine for beginners.
Remember, acrylic is very forgiving. If you don't like how something turns out, paint over it! Keep experimenting and learning as you go--becoming an artist is very experiential, and the most surefire way to find your own best technique is to keep painting.