Help Needed on figuring out payment schedule

[Brian Millanlebron]

Good Evening,

I have a question, so I bought a rental property with tenants in place, The previous landlord didn't require them to have a security deposit, How would I go by with telling them that I required a security deposit, also they pay at the end of the month for the month they been on, I would like to have them pay rent the month before. so for example have them pay the rent this month for the upcoming month and so on, how would I go about doing that. Because my issue is that since I bought the property this 15th they will be paying for may but I would like to have them start paying the rent for the month forward.