Before starting a retaining wall project, check with your local building codes. Soil conditions might need special attention during construction and a permit might be necessary. When altering a slope, the effects might run past your property line. Building a retaining wall requires moving some ground. If at all possible, use a terraced effect by building low walls on a slope rather than a single high one.
Since there is a substantial amount of excavating involved in building a retaining wall, hiring a professional who has access to equipment designed for this purpose would be a good idea. Drainage considerations, foundations, reinforcing rods and more make this a complicated job that is best left to the pros.