How do I cook pumpkin to use for pies or other sweets

I second the advice on using a squash instead of a pumpkin. It's so much easier and there is no risk of a stringy, watery result from using the wrong kind of pumpkin. I've used butternut, buttercup, and Kabocha squash. If you don't want to turn on the oven, steaming works very well for peeled sections of butternut.

I do recommend that if your puree turns out too wet that you cook out the excess moisture rather than straining, you lose a lot of flavor with the liquid and warm squash makes pies cook a little faster.
Also consider using other squashes, Kubocha squash are fairly dry and sweet and make a very good pie. The little "gold nugget" squashes would work well too.

I microwave the cut up pumpkin until it is soft. Do it on a low enough power and be patient with it. When cooled the skin pulls off easily. I then either purée it in a food processor or put it in a stainless deep bowl and go to town with an Emerson blender.

You can also peel and steam the flesh, though the roasting gives you the nicely caramelized bits.

Make sure you use a baking variety like "sugar" and avoid the giant jack-o-lantern type (those are bland, watery, and stringy). If you don't have the upper body strength to cut through you can try this:
http://food52.com/recipes/19323_pumpkin_pure_hack

Depending on how big the pumpkin is - cut it into quarters or eighths. Clean out the seeds and roast in the oven on a rimmed baking sheet @ 400 for about 45 min, or until the pumpkin is fork tender. When cool enough to handle, scoop out the flesh and puree in a food processor. If using it in a pie or cake, let it cool first