Easy. Find the leak, stop it!
Seriously though, finding the leak is half of it. See the above post.
You could use brake cleaner on a rag and wipe clean the area, let dry, and slop some gasket maker around it. But that's only a band-aid that might not even work. If you can get access to the leak, replacing the gasket is the only way. Don't use oil leak stoppers or additives.