How often do i need to change the water in my 5 gallon tank?

so im getting one betta fish and im gonna put in a 3-5 gallon tank. my mom said i can get one with a filter because she believes that that means u only have the change the water every month or even two months. i know thats not right, so how often do i need to change the water with a filter, and is it worth it even getting a filter for one fish in a 5 gallon. also, how often do i need to clean the tank if i have a filter, and do i really need a siphon? how do u really clean it step by step?