Choose only a small selection of basic colors in student-grade paint. The primary colors plus black and white will be sufficient. Mixing your own colors from the primaries is one way to achieve valuable understanding of the consistency of oil paint and color mixing. Also, as a painter you likely will be drawn to certain colors more than others. But as a beginner, you may not know what those colors are yet. Purchasing colors at random may prove to be a waste of money.
In your first paintings, or before you begin a painting, experiment with different consistencies of paint. Oil paint can be thinned with turpentine or medium. Oil paint thinned with turpentine is often runny, like turpentine itself. Oil paint thinned with medium is smooth and glossy, like oil.
Keep in mind the rules of fat over lean. Different paints dry at different speeds. Paint mixed with turpentine is thinner and dries faster (it is "lean"). Paint mixed with medium is the "fat", and it dries more slowly. The faster-drying paint should always go beneath the slower-drying paint. If the slower-drying paint was underneath the faster-drying paint, the top layer would crack when the under layer constricted as it dried.