Repair Leaky Shutoff Valve
This is not your normal gate valve so you might be in luck. Shut the water off at your meter. Clean off the valve and try to unscrew the larger of the two nuts to disassemble the valve. Support the body of the valve so you don't crack the solder joint. There should/could be a washer on the valve stem that you can replace. The smaller nut might have to be loosened and some packing/washer replaced too. 
If this doesn't work, then a valve replacement will be necessary. Replace it with a ball valve. Good luck.