Woolly buggers are the go-to beginner tie. It teaches you a lot of techniques, like getting materials to stay on top of a hook (marabou tail), running even bodies (chenille), turning hackle, and finishing clean heads without dragging junk into your knots. Throw in a bead head for some extra technique as Jeff said, or gets some non-lead wrap and weight the body to get the fly down.
I also like to tie little woolly buggers with oversized hackle and short tails and fish like a nymph. A few rainbow trout last weekend vouch for the tastiness of this.
Everyone should have a box of olive, black, and white woolly buggers in different sizes, so its a great fly to get a lot of practice on.