Paul,
You're asking big questions with just a few taps of the keyboard. While the answers to these questions can easily fill a book, I'm gonna try to give you just enough info to get you going.
> How do you know when it is ready to use?
Another great way to tell compost is done is that 98% of it is beyond recognition. It just looks like soil! Or piles of dirty twigs (twigs take a bit longer to break down).
> Is it necessary to keep the pile covered ?
Nope. If you become an expert, you might optimize things sometimes by covering it.
> I probably will have to wait until spring to get real serious about this but I would like to dabble until then.
Just start tossing everything into a pile. If it gets smelly, add some carbon (straw or leaves). Or you could just let it be a little smelly - most of the time if it's going to stink, you have to be almost standing in the pile to tell.
> can I put it on my lawn ?
Sure! Although you might want to save it for the most pathetic spots in your yard. It will then act as a sort of miracle cure.
> Are there things that should not go in compost ?
Most car parts would be bad.
Almost all organic matter is okay. If you put meat in there, you will attract a lot of undesirable mammals - so I suggest that you skip meats.