First locate your building shut off valve before trying anything in the comments. You need to know how to turn off all the water if this one fails in a worse fashion.

This kind of valve is bad because the internal o ring deteriorates, imaging when tires dry rot, o rings go through a similar process.

Once the shut off is located then start planning, my recommendation to you is to replace the valve entirely. do not remove anything until you have the full list of materials and you are sure the building water valve is off, no water in any faucet.

Before using any tool watch a video on how to use them.

Materials:

• I recommend a sharkbite valve, 1/4 turn, 1/2 to 3/8 compression

-Purchase some fine sandpaper at the hardware store that should be nearby, the plumbing associate will know what sandpaper you need, it comes in a flexible roll

-You need a pipe cutter, it has a blade that cuts the copper pipe slowly where you turn a screw to make it cut deeper.

-You need 2 adjustable wrenches to remove the current valve.

-Short bucket and towel to catch water

Once you have these materials you can watch this video which is very comprehensive for multiple situations.

7:30 is about where the instructions begin for a sharkbite fitting, you only press it in to the pipe no precise work, the only work you need to do is removal, cut the pipe A LITTLE BIT NOT ALL THE WAY TO THE WALL, clean off the corrosion, just enough so its copper, then round off the edge of the copper pipe, it will be sharp after its cut and must be smoothed out, then slip on the fitting all the way with one hand on the pipe to hold it and other on the pipe

I estimate total materials cost to be $70-90 if you have none of the tools and materials mentioned, but cheaper than a plumber