A retaining wall built from surplus concrete blocks. Note that the wall is battered and has no mortar.
A wall that is less than four feet high doesn't need a footer or mortar. It will stand for years with minimum maintenance.
Don't try to retain all of the earth with just one wall. Follow the grade and make different steps. Curve the wall to follow the natural lie of the land, this will mean a lot less work.
Here it is covered with flowers.
Nasturtiums are good for the first year. Later you can use any plant that will trail, English Ivy, Creeping Jenny, or Vinca.
If a retaining wall is not battered, that is if it doesn't lean back into the earth, then the earth pressure will push the wall over.
Take a look at retaining walls as you drive around and you will see many examples of walls that are falling over because they were built like this.
If the wall leans back into the earth then the weight of the wall counteracts the earth pressure and the wall remains stable. It also give your landscape a softer look.
Note that each stone is tilted back. That way the earth pressure has to lift the stones to move them. If you put each stone in level then they can still slide out.