Can you share a photo? What zone are you in? You can start with a ground cover and even some small trees to lessen erosion. If you have the muscle power or budget, you can terrace the area into usable space which will be easier to plant and maintain.
What is your project?
Are you looking to plant the slope?
As Naomie says it is difficult to say without a picture and more information because it depends on a lot of things as to how you would go about it. If it is a steep slope you will need to put something in it to keep the dirt from washing away from any plants you put in. You also want to consider how much work you want to put into it after it is planted as some plants need dead heading. Ground cover is a good way to start but be sure to check the plant to see if it goes with your amount of sun. Mine gets Sun all day and is on the south side of the house in southern Virginia so I have resorted to some hardy herbs that spread like a ground cover (oregano, mint etc.) I have also planted liropia (monkey grass) as that grows well here in red clay with lots of sun. I have also put in some trees for interest and to help stabilize the dirt. And, I am a fan of rock gardens so I have added some of those as well. Good Luck