how can I repair a leaking at the water main valve
Depending on your water supply, it may be as easy as calling the water company / department - depends where their responsibility ends and yours starts, in your local area. They might tell you it's your problem, they might send out their plumber to fix it at no additional cost to you beyond your water bill.

Otherwise, valve leaks or replacement can be easy or hard, depending on a lot of details it's hard to see from here. While a picture might help, you never know for sure until you start trying to take it apart. You might need the water company to shut off a valve at the street, if that's where their responsibility ends, so you can put the new valve on inside the house.