Step by step
Turn the water off that supplies this tub.
Remove the center "button" with the "H" on it by using a screwdriver.
Remove the screw under that button.
Remove the handle (might be difficult due to corrosion, so may have to use a penetrant spray, and wiggle the handle until it comes off
On the front of the bell shaped chrome escutcheon, there should be a round knurled nut. Unscrew that nut. Sometimes it is attached to the bell, so turning it will remove both.
At this point you need faucet wrenches. You can buy a set at the orange big box for around $10. They look like 4 inch long deep sockets. By trial and error figure out which one will slide on to your valve. Remove the valve and either replace the entire valve, or replace the small rubber washer that is attached to the inside bottom of that valve. Whichever you decide, be sure to bring the valve with you to the store so you get the correct replacements. You may also have to replace the valve seat, which is deep in the opening that the valve came out of. Using your finger, feel that valve seat. If it is rough, you can remove it by using a valve seat wrench. If you replace the seat and the valve, it will operate like a new faucet.