heart rate watches..... tips? hints? cautions?

I just bought one over the 4th to help figure out what I was burning when doing things like walking or hiking, or while I was at work. (I work 1 day per week so don't include my job in my activity level) Not sure which one I bought, its the $28 model at walmart. thanks.

Oh and is there a way to directly enter in calories burned per the heart rate monitor?