`require': cannot load such file -- spec_helper (LoadError)
You're running the specs from the spec folder. This messes up the load path. Run specs from the root of your project: ~/sheetal.
rspec spec/sheetal_spec.rbRspec adds the spec and lib folders to the load path automatically. If you're already in the spec folder, rspec is going to add spec/spec to the load path instead.
Assuming that you have your code in the lib folder, you would have to add both . and ../lib to the load path if you want to run your tests in the spec folder.