Sole P don't need to register with Fed, if you like

apply for federal ID, if you don't like, can use SSN,then file sch c on 1040. May need to register the business with state depend on state.
C Corp, S-Corp and LLC all need to register with state, apply for federal ID, all need to file entity tax return with Fed and state.
If you use your personal property such as house as collateral for business loan, C and S Corp still won't shield the liability.