should it be cooked, washed first ect?
You blanch it. Wash it, then throw it in a pot of boiling water. You could cut it up if that's easier. When it goes limp (not very long), drain it and let cold water run over it to cool it. At this point you can chop it and, depending on your future plans, press some of the excess liquid out of the leaves. Use freezer bags for freezing; the thinner ones are not as sturdy. The freezer bags can also easily be rinsed out and reused.
I successfully freeze my chard as follows for 1 bunch. Wash as you normally would. Bring a large shallow pan to a simmer with about an inch of lightly salted water. Cook only until just wilted. Shock in an ice bath: big bowl of cold water w/ ice. Once cool, drain carefully on clean kitchen towels. Either place in freezer zip top bags, or use a FoodSaver. I freeze in 2 person portion sizes...
I would do it as if i were serving it: wash it well, chiffonade it, blanch it. Then I'd toss it into a cold water bath to stop the cooking. Drain it VERY well -- even pat it dry, then bag it in zip lock freezer bags or use a Food Saver. Be sure to label it with the date you freeze.