I have a few questions about fish.?

yes you do have to change the water, even with a filter ... once a week you change a portion of the water with fresh dechlorinated water ... you need to cycle the tank with a form of ammonia so the right bacteria grows and makes the water safe for fish ... you can use plastic plants depending on what fish you get ... a betta would require either silk or live plants to protect his long fins ... most fish are tropical so yes you would require a heater ... there are cold water fish like a goldfish but your tank would be too small for goldfish ...