I have a ten foot slope that is the road embankment. It was a mix of clay, sandstone and shale and covered with weeds. The dynamic of a slope is that the water drains out of the top faster than the bottom, so it's important to add structure in the soil before planting. To deal with this, I ordered top soil, mushroom soil compost, and undyed wood chips. I mixed enough mushroom soil into the top soil to get a spongier structure to hold water better. Then I covered it all with wood chips. It's kind of like building a rain garden, but I wanted the water to stay where it fell instead of draining out of the top so quickly. The most mushroom soil was at the top, so I planted some azaleas since they like an acid soil.
The next critical step that I neglected at the time and know am sorry I did is to provide access for yourself. It's almost impossible to weed from the top of the slope. Therefore, you need a way to reach every part of the garden from the bottom. I have slowly created a couple of terraces and added some stepping stones, but I really should have terraced the slope before I planted.
My slope is planted in dwarf blooming shrubs, perennials and bulbs. I see it out my living room window, so I like lots of flowers. Over the three years I've been gardening the slope, I've added humus, worm castings, wood chips that decay into the soil and give structure similar to compost. I have a soaker hose watering system that is run off a manifold so I can get more water to the top of the slope where it dries out faster.
As to choosing plants, I would start by drawing up a measured plan and getting a soil test from your local Penn State Extension office. Once the test comes back, you will know what you've got to work with. Start by improving the soil and getting rid of large rocks, if any. Plants need enough room for their roots. Shallow rooters like ground cover can function in a few inches, and woody ornamentals can cope with their environment as long as you have an organic top layer. Some bulbs need six inches and some perennials need 8-10 inches minimum.
Once the soil is good and you've got some access pathways for weeding and maintenance, I would go to nurseries in your area and get recommendations for your conditions. It will help them immensely that you have done the earlier work. Once you select plants, keep wood chip mulch on the soil to prevent weeds from getting a hold. The project took me three years, but now my soil is so loose I can pull a dandelion straight out, root and all.