How do you build cash reserves when you are just getting started as a landlord?

[Mohammed Milord]

I am in the process of looking at single-family properties for my first rental. I don't have any cash on hand for the cash reserves. Is it important to have the cash reserves already saved up BEFORE you acquire a rental property, or is it okay to just set aside a percentage of your gross rent to fund the cash reserves after the investment property is acquired? How much money do you need you need in cash reserves for every rental property? I would appreciate any advice!