Pretty easy, really.
I go to Costco and get a big bag of mixed greens--the filed greens mix or sometimes it's called Eurpean or baby greens...--and it is a really good buy. the same thing in the grocery store costs much more per weight.
Then I chop up just about anything raw veggie: green or yellow squash, cukes, carrots, celery, grape tomatoes, all kinds of onions, cabbage, whatever.
Add your favorite dressing and you are good to go.
You can also start with baby spinach and use some of the veggies above, or use fruit slices, like apples, or strawberries, grapefuit sections, orange sections,
If you take you lunch to work, you can have the veggies shopped up in the fridge win plastic containers and play mix or match. Take along a small can of tuna or chicken or some cheese or some baked, chilled fish or chicken and a few crackers and you get to eat much better stuff than you can find at a fast food place.
I like Ken's Lite salad dressings. They taste great and my store often has them 2 for 1.