Check with your local nursery for some names of reputable landscape architects or contractors in your area. You want to be sure that any construction you do doesn't cause drainage problems.

You will most likely need an engineer involved. Drainage, pressure against retaining wall, risk of it cracking, falling over, calculation of rebar (if required) etc etc.

Different cities require different amounts of documentation for retaining walls. In LA if it's over 3' high we have to have a licensed landscape architect and an engineer and a permit process with the city. Then you need a contractor. Houzz has a find pros tab and it is searchable in areas so you may want to try a contractor or landscape architect in your area from here. Both should have connections to refer you to the appropriate people.

Thank you for the input. I am actually in Los Angeles and figured I would need an engineer, but didn't know where to start to find one. I was hoping a contractor or landscape architect would have the engineering contacts. Thanks for your help!

The California Building Code exempts retaining walls of 4 feet or less (as measured from the bottom of the wall to the top - not just the exposed portion) from needing a building permit. Anything taller than that will need a permit. Since you mentioned a wall of 5-6 feet high, when you add in another foot or 2 for what's below the ground, you are looking at a reasonably tall wall. I agree that you will certainly need Engineering to make the structure and drainage work. Your Landscape Architect should be able to bring in the right Engineer.

Good Luck!!