Picking is easy

For most vegetables, the best time of day to pick is in the morning. This is especially true for lettuce, herbs and greens. A pair of scissors comes in handy for clipping herbs, spinach and lettuce and cutting broccoli heads off, (the broccoli will grow again).
Swiss chard leaves can be twisted off at the base, like rhubarb. You'll hear the snap.
Cabbage requires a knife, cut at base, you might as well remove the roots too, because it won't grow back. For green beans, pull pods from plants carefully to avoid knocking flowers off of the plant.
Carrots will pop up a bit out of the ground, showing their orange crown, then they are ready to be pulled. Corn is ready when the tassles are brownish. Break off at the base of the ear and then grab firmly at the top of tassels and pull down to shuck, like a banana.
 For green onions, hoe around them to loosen soil, then pull the entire plant up. For regular onions, push leaves flat to the ground when the top turns yellow. Then harvest when the top turns brown.
For potatoes, when the plant tops die, dig up 8-10 inches from plant to avoid injuring the potatoes below. Dig deep and lift plant to pull potatoes off roots.