Take note of the geography, climate, sunlight, soil, and other limitations before you make your plan.
Step 1: Get the Lay of the Land
Look carefully at the slope of the land. Notice any steep parts and consider whether you are going to garden with the land, or whether you will do the hard work of leveling it out. A level lot is usually ideal and possible, but if you are on a budget, you will need to account for the extra labor and materials costs.
Step 2: Make a sun chart
It is important to understand where and when you get the sun and the shade where you plan to plant. Where you get a lot of sun, you should add sun-loving plants. Partial shade will require partial shade plants. The rest--the truly shady areas--will be great for shade-loving plants.
Step 3: Know your soils PH
Find out your soil's PH and what it is made of. Carefully research and consider the plants you wish to incorporate into your garden. Do the research to find out what PH they prefer. If the soil is too acidic, add some limestone. If the soil is too alkaline, add some sulfur. The ideal PH depends on what you plan to plant in that location.
Step 4: Know your soil's content
Do some digging to find out if your soil is clay, loam, or sandy. Clay soil drains slowly and may drown some roots. Sandy soil allows for quick drainage, but it dries out more quickly. Loamy soil is usually ideal; it has larger particles, drains well, and makes digging easier. Feel free to suplement your soil where you want it to be, but do your research as adding aggregate, etc. may change the PH of the soil as well.
Step 5: Consider drainage issues
Make sure large amounts of water drain away from your house or any neighbor's house as well. You do not want to flood any basements. Make sure you are in accordance with local regulations about where stormwater can and can't flow.
Step 6: Make sure your plan doesn't break any regulations.
Cities, towns, and master-planned communities often have strict requirements about what you can and can't do. Make sure you are aware of these before breaking any ground. If you are subject to an HOA, check the CC&Rs and have your plan approved before beginning.
Step 7: What is your climate and growing zone?
Make sure your plants and landscaping are compatible with the climate. Consider the long-term availability of water before choosing thirsty plants.
Step 8: Look at the surrounding architecture
Make sure your garden's appearance and style will complement the local architecture. Otherwise, your garden (or your home) may feel out-of-place.
Step 9: Consider the wildlife
What organisms do you wish to attract to your garden? What organisms do you hope to deter from your garden? Consider the benefits of the bees and other pollinators. Also, you will never regret making your garden attractive to hummingbirds.
Step 10: Think of your needs
Just because it looks good in a magazine doesn't make it the best for you. Make sure it is the ideal place that you will enjoy. Make it something that brings you peace and happiness.