The easiest would probably be a pre-fab retractable awning - either manual or motorized. They are pretty simple kits that are available at the big box stores.
All depends upon what your trying to do. Are you trying to make it weather tight, or simply block out the sun? As Steve said a awning works well. You can put a gable type, shed type, mansard type or simple hip type. Each type has their own construction issues. Is the deck attached to the house? Perhaps the shed type would be best.
If you are going to do this yourself, make sure you investigate how to use flashing to make it water tight where it joins your house. We had a deck and when I wanted a screen porch, we used the slanted style. It is more than 8 feet tall where it joins the house and slants at about a 15% angle toward the back. That way you only have to deal with rafters and do not have to get into joists. Seal the inside with outdoor plywood and box in the eves for a finished look. My sister did not screen hers but used 4x8 sheets of lattice laid lengthwise to for a 4' tall privacy railing.
the easiest would be a lean to roof anyone could build that
Just had a dear friend ask me the same thing about putting a roof on her deck. Pretty much any shed roof will work, but the most important thing one needs to understand is that the roof needs to be supported properly. Way to many people just put a post on their existing deck, whether is be a cement patio, or a raised wood deck, It is doubtful that the foundation supporting it will be enough to properly support a roof. Even more so up north were we need to worry about snow loads. So before I can even tell her that we can do this, we need to check out the footings for her existing deck to see if this would even work, or would we need to dig bigger footings to support the load of the roof. So if anyone is looking to do this, and its not a very hard project and quite fun to do, be sure to check the foundation that will be supporting the roof out and away from the house.
Woodbridge is very right! Check out foundation. Also be sure you check the "banding board" to make sure it is solid with no rot and secured correctly to the house. And any posts for the roof should be anchored into the house also with large bolts! Adding a roof is more than just opening an umbrella!
Debbie, there are more suggestions waiting for you here: http://www.facebook.com/Hometalk/posts/115344141937824?ref=notif¬if_t=share_comment
Put a tin roof p. If you like the sound of the rain, it will be amplified, but sound so nice. Tin roofs come in many colors now, so you should be able to find something you like. Good luck.