Beverly, Your showerhead and tub spout are connected to the same pipe. When you toggle the valve lever on the spout (the diverter valve) you redirect water from there to the showerhead. Once in the redirected position this type of valve often leaks to some extent. However, if you are experiencing excessive leakage you may want to replace the tub spout thus ending up with a new valve, and possibly, less leaking. Cleaning your showerhead also should help. Improving the head's ability to freely pass water can help. Clean the showerhead by filling a plastic bag with straight vinegar (any kind). Place the bag around the showerhead like a feedbag for a horse. Fully immerse the head in the vinegar. Tie the open end of the bag in place around the shower arm with a twist tie or a rubber band and let sit for 24 hours. Run the shower for about a minute once you remove the bag. It should run more freely. To replace the tub spout you must first determine whether the spout is held on to the pluming pipe with a threaded connection or if it is a slip connection. Look at the underside of the spout. If there is a screw then remove it and the valve should slip right off. If no screw is visible anywhere on the base of the spout then simply turn it counterclockwise and unscrew to remove it. If a screw exists and you can't seem to slip it loose try unscrewing it. Some have both a threaded connection and a retaining screw.

There is no better diverter than the kind that works exactly like the hot- and cold-water valves themselves. Single valves are available for this exact purpose. Unfortunately, installing such a valve at this time could be expensive since you would have to access the wall interior to perform the installation.