To me, personally I like heaters that I can adjust outside of the tank, such as catalinas > http://www.catalinaaquarium.com/stor...x.php?cPath=42
I have a few of those.
You need a thermometer for checking temperatures. Check reviews too before you buy, some are bad and off by 2-4 degrees.
To adjust him to temperature, you need to set up the tank, put in heater, set temperature to desire. After temperature is set and stable, with a small cup or tank or bag with fish in there, let it sit in tank for til they are similar in temperature. If you don't know, 30min-1hour is usually sufficient for me. And that's about it. Good luck. :)