Creating a lovely garden is certainly possible without a plan. Mother Nature does it all the time. However, a plan will make sure your garden fits the needs of your family at a budget you can afford. Your garden plan should grow as your plants grow. Keep both fresh and green.
Your garden doesn't exist in a vacuum. Take some time to see how much sun it gets, where the prevalent winds blow and what areas are shady. If there is a slope, look at how the water drains and where. Use a hose and sprinkler if there isn't any rain in the forecast. Make notes of trees and bushes. Look at what views need to be blocked. You don't want to look at the neighbor's garbage cans from your garden. Borrow views and highlight them. Make your yard look bigger by borrowing a pleasant view of a park, greenbelt or lake and framing it with trees and bushes.
Drainage is an important feature for thriving plants. Plan to build up boggy areas with raised beds. Add French drains to direct water away from wet areas to dry areas. Make sure that water drains away from the house and garage. If you're planning a pool or pond, drain water away from it. You might think a pond in a boggy area would be perfect. The problem is that the pond will catch runoff from the rest of the yard that has fertilizers, herbicides and pesticides.
Different families have different priorities for their garden. If you have small children, a play area is important and you might want to forego a water feature for safety reasons. Knowing what you want from your garden is important to enjoying the garden. Think ahead to the future as well as planning for the present. For example, add a grass oval to the children's play area and a round sandbox. Later when the children are older change the oval to a putting green and install a fountain where the sandbox was.
Incorporate What's There
Mature trees are valuable and take years to grow. Use what's currently in the garden rather than bulldozing the area and starting fresh. That tree might not be exactly where you want it but moving it is expensive and risky. Work around the tree with a patio or plant a shade garden under its boughs.
Keep a Journal
Sketch out the backyard as it is now and as you would like it. As you make changes in the garden keep a journal of where you planted what. Take photos and make notations of plants that successfully bloomed and others that didn't do quite as well as you expected. Keep the tags and labels of plants you've bought so you know what to buy again.