Sloped areas make landscaping and gardening more challenging, but don't let that curb your enthusiasm. Your will, combined with a little information, will pave the way. The age-old approach to growing on a slope is the garden terrace. This may sound like a massive engineering feat, but if done in miniature as a small planting shelf, it's actually quite feasible. A good entry level terracing project is to build a small flower bed around an existing tree. This makes a focal point along the slope and can give you the confidence to try bigger projects later.
Remove any existing vegetation on the downhill side of the tree. Clear an area roughly 6 to 7 feet below the tree and 5 or 6 feet to either side.
Dig a shallow trench across the slope about 6 feet below the trunk of the tree. The stones that will used to build the terrace will use the trench as a stable base. Center the trench on the tree, and dig it about 3 inches deep, 8 inches wide and 8 or 10 feet long. Make the trench curve up into the slope in the shape of a smiley.
Spread an aggregate paver base material in the trench about 2 inches deep. This is essentially a finely ground rock material or very coarse sand and will make a firm foundation for the bottom layer of stone.
Lay the first layer of stone. Shimmy the stones into the aggregate base material an inch or so as they are placed. Choose some of the larger rocks for your first layer, selecting ones with complementary shapes that fit snugly together. Place the stones so the long side is perpendicular to the direction of the slope.
Continue laying the stones in horizontal bands. The first band will be the shortest, centered at the low point in the trench. Each successive band will be longer, as the wall is following the curve of the trench into the hill. Place each course of stones slightly closer to the slope than the one below, so the wall leans slightly into the slope as it goes up.
Add topsoil behind each course of the wall as it is built. This supports the wall from behind and makes a deep, rich planting bed. Build the wall until it is up to the base of the tree trunk or to a maximum of three feet high -- whichever happens first. Continue to add topsoil behind your completed wall to make a mounded bed between the trunk of the tree and the top of the wall. Never cover the base of the trunk with soil.
Plant according to the site conditions and your personal taste. If it's a large tree, choose plants that can tolerate full shade. Smaller trees may not shade the planting bed at all or provide partial shade, depending on their size, so make sure to select appropriate species. Use taller perennials or low shrubs in the back of the bed and smaller flowering plants in the foreground. Be sure to use a few trailing plants along the top of the wall itself -- these will look lovely cascading over the natural stone. Succulents are a good choice, especially if it is a hot, south-facing slope.
Cover the planting area with 2 or 3 inches of mulch and water thoroughly.