Since you should be eating at least one salad a day, you have to stock up on the makings, and keep refreshing your greens as they don't keep all that long. Living alone, I still manage to have two heads of lettuce and a bag of those pre-washed greens in the fridge at all times.
To make a salad, I usually make enough dressing to last a couple of days, but it's better to avoid the temptation to make a big jar. It's easy enough to make it as needed, and the results will be much better with a fresh dressing.
I take a little bowl and dump about a teaspoon of Dijon mustard into it and add about 1/4 teaspoon -- no, maybe less than that -- of salt on top. Using a fork or whisk, I begin stirring the two as I add about 1/8 of a cup of vinegar. When that is well mixed, and still whisking, I dribble enough good extra virgin olive oil to suit the dressing of the day. Measurements depend on my needs of the moment, but it's roughly three parts oil to one of vinegar, always with a little mustard to emulsify the mixture. Everything else is up to you. Sometime a drop of honey is warranted, and I add more mustard if I want a decidedly honey-mustard flavor. I got that trick from watching Bobby Flay. Sometimes I add herbs, fresh or dried, and I almost always smack a clove of garlic and throw that in -- unless it's honey-mustard.
Different oils and different acids can add interesting taste. If I want a dressing for a salad of mixed greens, walnuts, and fresh orange sections, I might use canola oil combined with walnut oil, and lemon juice in place of the vinegar. Usually I want my salad to taste of Italy, but this one has a decidedly French tone.
I once made a dressing I got from O Magazine which was very low in oil and substituted orange juice concentrate. It was delicious.
Some people contend that what makes a salad is the little touches: One of my husbands thought every salad should have some fresh onion in it (I use red or Vidalia for this); my daughter adds toasted sunflower seeds; and capers, feta cheese, or crumbled bacon are special favorites of mine. You don't need to add all in every salad; in fact, your choice of any one from the above list, or one of your own, will make your salad memorable.
My diet book exposed me to a very delicious salad, which I'll share with you another day. Wouldn't you like to tell me what you think makes a salad?