Depends on the knife and your skill. With more expensive knives the edge is harder and lasts longer. For those a steel will keep the edge in shape, often for many months if you treat the knife well. With cheaper knives you will dull the edge quickly and need to actually sharpen the knives pretty frequently.
For most people I'd recommend Chef's Choice sharpeners. They make some easy to use and effective sharpeners. The manual ones with rolling diamond grit wheels are my favorite. They will cost more than inexpensive knives but will give a decent working edge with minimum effort or skill. The company also makes motorized sharpeners. Those are fast and more expensive. They will also remove a lot of steel from your knife if you don't use them carefully.
Carborundum whet stones are cheap and will work, though they require much more skill to use. Chef's will use more expensive Arkansas stones or Japanese water stones. Those give the best edge but require quite a bit of skill to get full benefit from. Those only make sense to use with better quality knives.