Answer: Welcome aboard! Let me warn you....gardening is addictive, so your 6'x6' bed may soon expand to take over your whole front yard! If your garden receives direct sun all day, you can grow any number of sun-loving annual flowers. It is a good idea to start off with the easier types to build your confidence: try marigolds, zinnias, cosmos, lantana, portulaca, and verbena. In fact, you can try most any in your new garden!
You are wise to think of improving the soil first -- you're already thinking like an experienced gardener! It's always a good idea to add organic matter to any garden soil. Organic matter improves drainage on heavy soils, increases water-holding capacity on sandy soils. Incorporate compost, shredded leaves, grass clippings, well-rotted manure, or other organic matter directly into the soil when you first turn it over in the spring. (Wait a few weeks to plant if you use non-composted additions.) Throughout the season, use grass clippings or shredded leaves as a mulch--it will keep down weeds and add even more organic matter.
There are many great books out there on beginning gardening. I suggest that you browse at your bookstore to find one that suits your region and your gardening aspirations. Best of luck to you!
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