metric feets and poetic forms....

Are a lot of people using the metric forms around here, like Iamb, Trochee, Anapest and Dactyl? I've tried to understand this thing with stressed and unstressed in these feets but I still don't get it. Could someone maybe explain clearly how to know when a line is stressed or not?
(...sidenote: sure it's ok to write without any forms but I feel that I want to master these forms before I start breaking them...)

One more question: Wich is your favourite poetic form?
-I just love a good triolet