watt

Hello, I'm trying to find a heater for my new 10 gallon tank, but I'm not sure what I should go for, what watt should I go for, I've seen some that go from 25 watt to 200 watt.

I'll probably be going to Walmart or Petco for tank supplies later on, so I'm wondering, is 50 watt any good for a 5 gallon tank? And what kind of common brand that's not too expensive would be good? Do most heaters turn off and on by themselves to regulate the temperature steadily? Two more...