Yes you will be bullied for not meeting our standards but afterwards you will realize how rational our standards are. (Don't be a bigot and always side with the demise of the american empire you'll be ok for a while.)
You can be a leftist. You don't have to "be" better but you do need to think better than "unions are good" and "healthcare is a right." Leftism is a rejection of capitalism, and a demand for true equality and the end of private ownership of the means of production.
As far as theory goes start with the Communist Manifesto. Its a really easy read and if you agree with it you are probably a leftist. There are Anarchist manifestos that would also be considered leftism but disagree with Marx on some things but I'm not an anarchist so I can't point you in a good direction. Deeper theory will hopefully make you a better leftist but it won't change the core idea.