Monarch butterflies eat nectar from flowers, just as other butterflies do. Butterfly mouthparts are made for drinking nectar. If you look at a monarch butterfly's head, you will see its proboscis, a long "straw," curled up below its mouth. When it lands on a flower, it can unfurl the proboscis, stick it down into the flower, and suck up the sweet fluid.
If you're planting a garden for monarch butterflies, try to provide a variety of flowers that bloom throughout the months when monarchs visit your area. Fall flowers are particularly important, as migrating monarchs need plenty of energy to make the long journey south. Monarchs are large butterflies, and prefer larger flowers with flat surfaces they can stand on while nectaring. Try planting some of their favorite perennials, and you're sure to see monarch all summer long.
Monarch caterpillars eat the leaves of milkweed plants, which belong to the family Asclepiadaceae. Monarchs are specialist feeders, meaning they will only eat a specific kind of plant (milkweeds), and cannot survive without it.
Monarch butterflies gain an important defense against predators by feeding on milkweed as caterpillars. Milkweed plants contain toxic steroids, known as cardenolides, which are bitter-tasting. Through metamorphosis, the monarchs store the cardenolides and emerge as adults with the steroids still in their bodies.
The caterpillars can tolerate the toxins, but their predators find the taste and effect more than unpleasant. Birds that attempt to eat monarchs will often regurgitate, and quickly learn that those orange and black butterflies don't make a good meal.
Common milkweed (Asclepias syriaca) often grows along roadsides and in fields, where mowing practices may cut down the milkweed just as the caterpillars are feeding. Butterfly weed (Asclepias tuberosa) is a showy, bright orange perennial that gardeners usually prefer for their flower beds. But don't limit yourself to these two common species; there are dozens of milkweed varieties to plant, and monarch caterpillars will munch them all. Monarch Watch has a nice guide to milkweeds for adventurous butterfly gardeners who want to try something different.