The lid and gasket keep dust and debris out of the engine while keeping the oil in. It's not uncommon for the gasket on an older engine to develop a leak, and in most cases this problem is not serious. Replacing a leaky gasket is a relatively minor job, but it's definitely something that should be taken care of. Also, you will need the proper torque specifications for reinstalling the valve cover. The basic steps are outlined below.
Pliers Allow the engine to cool before beginning work.
For safety, disconnect the negative battery cable. Tuck the hose back out of the way. Use a socket wrench or speed wrench to loosen the cover bolts that hold the valve cover in place. Lift off the valve cover. If the cover does not come loose easily, tap it gently with a rubber mallet to free it from the engine head.