Gardening Zones - First, figure out what zone you are in. Do a Google search for USDA Plant Hardiness Zone Map. You will need this information when you get ready to shop for plants and you'll need to know when it is safe to plant or the last frost date for your zone.
Garden Size - I don't always heed my own advice, especially on garden size, but I will go ahead and say to start small! Whether this is your first garden or your tenth, it's so much easier to start small and then add on the next year if you want to go bigger.
Selecting & Clearing A Place for Your New Flower Garden - Next, find a spot for your garden. Use a garden hose or rope and lay it out to mark the edge of the flower garden. Now, clear out all the grass and weeds. This is hard work and another reason to start small.
If you're in no hurry to finish your flower garden, rather than digging out all the sod and weeds, I would highly recommend what is called, the "lasagna method." This is basically where cardboard or thick layers of newspaper is laid on top of the area where your new garden will go, and then covered with layers of grass clippings, shredded leaves and compost. The area is soaked down with water and left for several weeks to kill off the grass. I have even done this in the fall so the area is ready to plant the next spring.