A ground or support is any kind of surface you can paint on. The choice of ground is in quite important because it determines in part how your painting will be perceived by the viewer.
Your support must satisfy a minimum of conditions. It must accept the paint well (ie, the paint must stick in one stroke) without absorbing too much of the pigment.
The least expensive oil painting ground is treated paper. It is very useful for practice and even for doing certain commercial work. You can also buy canvas pads. These pads usually contain 10 sheets of medium tooth 5-oz cotton canvas that has been triple-primed with acrylic gesso. The tooth of paper or canvas is the degree of roughness or smoothness of the canvas. You can tape the oil paper or canvas pad paper to a drawing board. Make sure it is secure and does not shift when applying brushstrokes.
Next in line are the popular canvas boards or panels which are sheets of cardboard covered with inexpensive white painted cloth. These panels are very popular because they are inexpensive (especially when bought in bulk), easy to store, and easy to carry outdoors. They are however not permanent, ie, they will deteriorate over time.
Good results can also be obtained from un-tempered Masonite or 3-ply chipboard prepared with three coats of gesso on the front and one coat on the back to prevent warping.
You can also use so-called museum board which is on the order of good-quality mat board. This board is quit absorbent but is inexpensive to practice on.
The ultimate ground for oil painting is canvas stretched over a wooden frame. It has wonderful elasticity and resilience, and history has shown that it has very good permanence. Canvas is of course more expensive but when you're ready this will be your ground of choice and you'll never want to go back to anything else.
You can buy commercially pre-stretched canvas. In fact, there are plenty of brands, sizes, weights, and qualities to choose from. Only experience will teach you which type of canvas is best suited to your style and subject matter.
Canvas cloth is either cotton or linen. The finest canvas and most expensive is made of linen, which stretches better and has a better tooth. Cotton can be a bit difficult to prime.
Look for cloth with an even weave. The canvas texture can be tightly woven and smooth to fairly coarse with an open weave. That means the tooth of the canvas can be fine or coarse and anything in between.
If you paint a lot, even commercially made pre-stretched canvas can become expensive. If so, you can buy rolls of primed or unprimed (raw) canvas. Then with stretcher bars you can create a support of a certain size. If you bought unprimed canvas, you still have to prime the canvas with an oil-based primer.
To save money you may be tempted to work on small canvases. This is not recommended. Unless you are an experienced artist, working on a small ground can easily result in tight, overly controlled paintings. So use subsidies of at least 16 "x 20".