Q: How can I clean my 21.5" iMac screen?

It depends what the streaks are from. I would just use a very lightly dampened washcloth. Windex and similar products don't work as well.

From: How to clean Mac products:

Displays

Disconnect the display from power, from its connection to a computer, and from any external devices. Next use the cloth that came with your display or another soft, dry cloth to wipe dust from the screen. If additional cleaning of the display panel or case is required use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don’t use window cleaners, household cleaners, aerosol sprays, solvents, alcohol, ammonia, or abrasives to clean the display.

Warning: Don’t clean the screen with a cleaner containing alcohol or acetone. Use a cleaner intended for use with a screen or display. Never spray cleaner directly on the screen. It may drip inside the display and cause damage.

Follow JG's suggestion, with the addition that you should use a microfiber cleaning cloth, the kind meant for cleaning eyeglasses. Clean in sections. I usually apply water to a small area of the screen first using a dampened Q Tip, then finish cleaning and drying the area with the microfiber cloth. I don't think Apple includes these any longer, but the small black one that came with my late 2009 iMac is excellent.

If it's really serious, then some preparation meant for these Apple screens shoud be used. I've seen iKlear promoted, but I don't know how safe or effective it is for the iMac screens.