If it's a whole bone-in ham, lucky you. That's one of my favorite treats. We do those for Christmas every year.
We have one of those big electric roasters that we use, as I usually have other things going in the oven. Kim trims off some of the excess fat, and adds a little water to the bottom of the pan, which actually steams the meat, making it very tender.
I've also done a lot of them in the oven, as Gretchen suggests, although I like to score the fat on top and tent it with foil for the first few hours. Then I remove the foil, baste with a glaze and finish cooking uncovered to crisp up the skin. I'm ashamed to admit that I love to pick off bits of crispy skin and fat to nibble on.
Be sure to save the bone, some of the skin and fat and some scraps of the fat for a big pot of ham'n beans or other dish...black-eye peas, split peas, crowder peas, green beans, lima beans, greens, lentils, etc.