Whenever you want to use in a dish mussels that are still in their shells (live mussels), use preferably fresh ones, although you can also use the ones sold packed in water (vacuum-sealed). Make sure though you check the packaging date and prefer the ones that have been packaged more recently.
To clean mussels in their shells, you will need a large colander and a brush (or small knife).
I am very meticulous when it comes to cleaning any kind of shellfish.
Place the mussels in a large colander and rinse them very well under running water for 3-4 minutes to get rid of the sand and any impurities. Discard any mussels whose shell is broken.
You will notice that there will be mussels whose shell is tightly closed and others whose shell is open. Those whose shell is closed are safe to keep. You need to tap on the shell of the ones that are open and if they close, then you can keep them. If not, discard them, they are dead.
Note: Not all mussels will close as soon as you tap them so put them aside and check them after 1 minute; if they're still open, discard them.
Grip the beard of each mussel and pull away towards the hinge end of the mollusk (not towards the part where the mussel opens up because that will cause the flesh of the mussel to tear). The beard is a mass of tough, brown fibers that stick out between the two shell halves of the mussel. Sometimes the beard is tough and thick and others it is thinner and it helps the mussels (as well as other mollusks) cling and attach to rocks or other objects in the water.
If you're having trouble pulling away the beard with your bare hand, use a knife to assist you in the tugging.
Give mussels a good scrub with a brush (or a small knife at a 45 degree angle, 'scraping' the shell), removing any barnacles that are attached to their shells. Then give them one last rinse and let them drain in the colander.
They are now ready to be used in your dish.
By the way, did you know that the orange flesh of the mussels indicates that it is a female while the off-white, paler flesh indicates that it is male?
You will find this helpful in the following recipes:
Greek Mussel Pilaf or "Midopilafo"
Mouclade - French Steamed Mussels in a Curry Cream Sauce