Faith,
I live in northern Illinois, too. We grow a large vegetable garden every year but not much for flowers. I’m not good at visualizing a good flower garden so I stick with whatever grows well on its own without too much interference from me.
Things that are easy to grow and hard to kill in northern Illinois would include: oregano, basil, sage, violets, Johnny Jump-Ups, petunias, any wildflower seed mix from a local store, sunflowers, rhubarb, asparagus, zucchini squash, butternut squash, Black-Eyed Susans, milkweed (great for monarch butterflies), lilacs, and red osier dogwood. All of these plants take a minimum of care, mostly just watering them, if that.
Here are some tips for successful gardening:
Wait until all danger of frost is past. Usually, after May 15 here in northern Illinois.
Harden off any plant you buy or grow indoors from seed on your own. This means to gradually increase the amount of time the plant is outside. Start in a shady, sheltered spot for a few hours and gradually increase the amount of time, breeze and sun the plants are exposed to. Make sure plants stay moist. Plants in pots tend to dry out very quickly and need to be watered more frequently than you may think. After about a week of increased exposure to the sun, they are ready for the garden.
Once the plants are in the ground, keep them well watered by making sure they have about an inch of water each week. If it doesn’t rain, that means you will have to water them about twice a week. Let the sprinklers run at least two hours each time you water. And be sure to water as early in the morning as possible.
Weed at least once a week. Mulching with straw or compost will cut down on the weeds but you will still have to do some weeding. Mulching is one of the big secrets for success. It helps retain moisture, protects the plants from soil-borne diseases, and keeps the soil temperature even as well as keeping weeds in check.
For tomatoes, pull off the bottom set of leaves. Plant in a hole deep enough to come up to the next set of leaves. This will give the plant a good root system. Pinch off any buds, blooms, or fruits that are on the plant. You want the plant to put its energy into making roots at this point, not fruit.
Feel free to ask any more questions. You may want to get a good gardening book such as Square Foot Gardening.