The best gardens result from a plan -- a solid design that plans for beauty and function. First, measure your plot and make a sketch (to scale) that considers all structures on your land (such as swings, garages, decks, patios, and driveways. Plot out tree positions too, as you want to garden around them. Make a note of how much sun and shade your plants will receive. Now for the dirty work: assessing the soil conditions. Kits are available to help you test the pH and nutrition of your soil. Is your soil very dark and rich? Black earth retains moisture and is home to worms. If your soil is sandy or clay-like, you will have trouble growing plants. An option, in this case, is to order topsoil.
If you have a family, there is the additional responsibility of planning for how pets and children will affect the garden. A good amount of grassy area should suffice for the children's play. Make sure garden boundaries are thick so as to keep out pets and kids, who might walk all over the newly sowed plant life.
Begin by planting hardy trees and shrubs. Plant the largest things first -- it's easier to add the little plants later. Try to avoid plants that are invasive and will stunt the growth of plants around them. Will the smaller plants make a culinary garden (tomatoes, carrots, herbs) or a flower garden?