I bought some frozen Green Mussels and would like to know how to prepare them please. I'm not even sure if I need to defrost them first.

You should make sure the shells are closed if not tap them, if they close they are ok to use if dont close discard. Wash them and take the beards off (the stringy bits that on the shell). Good recipe is to use LLoyd Grosmans Tomato & Chilli sauce, lightly fry chopped onion and garlic, pour in a little white wine and the tomato &chilli sauce, heat till hot (of course) and then put mussels in, put lid on pan for about 3 mins until mussels open. Delicious.. Enjoy