Choose your site and check the soil.  Do you have good drainage?  Do you have clay or sandy soil?  Most likely you will need to amend the soil by adding good organic matter (i.e. compost, mushroom compost, animal manure, leaves, etc.).  Make an outline of the garden.  Lay a hose around the area in a pleasing way to mark your border.  Draw your plan on paper.  Determine the climate zone where you live.  It is most likely Zone 5 in the Chicagoland area.  Pour over catalogues and magazines to decide on the plants for your garden.  Are they for sun, shade, or part shade?  What colors do you wish to use together?  Look at the color wheel.  It may give you some ideas.  In your planning, think of tall plants at the back, moving forward to smaller ones in the border.  You may want to have bushes and vines in your plan.  Perennials are plants that come back for many years, but usually have a specific bloom time.  Annuals are plants that live one season, but usually bloom all summer if deadheaded.  Bulbs are also a must.  There are some for spring, summer and fall.  Biennials are plants that usually have leaves the first year and bloom the second.  Keep leaf color in mind, as it is an important part of the look in your garden.