Why do you keep paying them and why the secret service?
Sorry, that information is normally not publicly available until after the sitting president has left the office.
Are taxpayers paying for Malia's Mexican vacation and Secret Service protection?
He only has one and it is Timberwolf.
Do the Secret Service have to go to the bathroom with them?
In regards to the President's security detail, essentially yes. Where ever the President goes he is accompanied by a certain number of secret service agents.
How do you join the secret service and them become a presidents bodyguard?
Until they die or refuse it. Also, their spouse is protected.
Do presidents who resign keep secret service?
I don't believe the Secret Service protect former-Presidents. However I am aware of private firms being hired by their home-state to provide protection.