how can i make a half cylinder? here is a picture of the shape i would like to make in blender i have no clue how to even begin making it. what i want to know is how to make this shape in blender so i can use it for a project im doing. i tried adding a cylinder but then i could not figure out how to get the cylinder to be cut in half.
A perhaps simpler way is to use the cylinder primitive. 

Add that to your scene, 

then in wireframe (Z key), and from the front view, 

delete half vertices:




then rebuild top/bottom faces selecting the edge loop and pressing F key




on both sides

then you get



Another way could be to use a boolean modifier, too.