Basically, as @RegularRival said, your object’s Normals (the faces) are inside out. If you’re using Blender 2.79 (or earlier), Ctrl + N “Makes Normals Consistent” inside:
If your meshes faces are already inside, it’ll flip them all to the outside.
However if you’re using Blender 2.8 or later (which honestly, you should be since it’s better to use and learn on), Ctrl + N starts a new save file, so the solution is to go into Mesh > Normals, > Flip Normals
