The most appreciable element of a piece of wood is the pattern of its grain. The whorls and swoops of the wood fibers draw both the eye and the hand, compelling us to run our fingers over the smooth but patterned surface. Some woods are fine in their natural state, but others benefit from a stain. Both dye and pigment-based stains can deepen a wood's color, highlight its grain pattern, and otherwise improve its appearance.
Before you stain, bring an example of the wood on which you'll be working to the hardware store. Don't rely on the sample piece displayed in the store; a single stain may exhibit subtle or dramatic differences in appearance once it dries, depending on the type of wood. Cherry wood, for instance, has hard and soft areas that bring about different results. The natural color and condition of the wood, as well as the effect you want to obtain from the stain, dictate which type to use. Pigment stains, which add color by filling the pores of the wood, are lighter shades; dye stains, which are absorbed by the fibers, tend to be darker. Dye stain works well on wood that has been mildly marred with sanding marks or other imperfections, while pigment stain may accentuate blemishes.
Once you've chosen a stain appropriate to your wood, sand the wood, in the direction of the grain, with progressively finer grits of sandpaper -- from 180 to 220. Each component of a piece of furniture needs to be considered as a separate entity; for example, a table leg's grain may run in a different direction than the tabletop, while the feet may run in still another direction. After sanding, the surface should feel silky to the touch, with no visible sand marks. Rub the wood with a lint-free cloth dampened with paint thinner, and then pat the surface with a tack cloth (be sure to just pat and not rub, so no stray particles of wood dust or lint are pressed into the pores). Apply a coat of wood conditioner to the piece, and when it dries (after approximately 24 hours), resand very lightly with 220-grit sandpaper.
With either a rag or brush, apply the stain, always moving with the grain. After the stain has dried thoroughly, you may want to apply a coat of varnish, especially if the furniture will be used a lot (for instance, a coffee table). A water-based stain should be finished with a water-based varnish, just as oil-based stain should be finished with an oil-based varnish. The easiest finish is probably wax. Clear wax will offer the wood some protection and still let the natural beauty of the wood come through, while an amber wax will add another tint to the piece.