Homemade salad dressing is so easy to make, and so much better than the store bought variety. A simple vinaigrette can be made with oil, and anything acidic, such as vinegar, or citrus juice.
As a baseline, dressing should be 3 parts oil, to one part acid. Depending on the ingredients, and what the dressing is being used on, that might need to be tweaked a bit. Personally, I tend to err on the side of extra acid, preferring a little more tang to the dressing.
To make the perfect salad dressing, start out with a good quality oil. Olive oil is the most common base for a vinaigrette, but other clean, flavorful oils such as walnut can also be used. Vinegar, or citrus juice can provide the acid.
There are lots of types of vinegars on the market. Balsamic vinegars can range from tart, to syrupy and sweet, depending on how long it has been aged. There is even a white balsamic, that has a milder taste. Red wine vinegar is a bit more tart, and half wine vinegar and 1/2 balsamic can be a nice blend. There are also flavored vinegars which add an extra element. Lemon juice is a nice alternative to vinegar, as is lime or even grapefruit juice.
One of my favorite tricks is to make the dressing in a dijon mustard jar that is almost empty. Just pour the oil and vinegar or lemon juice into the jar, and shake it vigorously. The mustard left on the sides of the jar will mix in and add another level of flavor to the mix.
Once you start making your own dressings, you will never want to go back to bottled versions, which are laden with preservatives and other unnecessary ingredients.