How can I change a valve that leaks at the joint?

If that is the main shut off for the water supply from the street to the house you will have to contact the water department to have them shut off the water supply at the street before you remove that valve or proceed with any repair.

The orange handle is for a quarter-turn valve that shuts off the supply. Those serrated nuts on either end of that valve are for removing that valve. That big part on the right side may be a union nut. The other joints may be threaded or glued (can't tell from the picture). Also, I do not see the white strip you are referencing.

Before you try any repair you need to know just what that valve is for and how to turn off the water supply to it if necessary. If you shut off that valve any down stream repair can be made and the fittings removed but not on the supply side without shutting off the water supply. I hope this helps.