1. Most likely leaking from the (large) union nut which connects the radiator tail to the valve outlet.
2. Given that the pipe up from the floor looks crooked, may also be leaking from the compression joint between the pipe and the valve inlet.
3. Valve needs to be removed and either replaced or cleaned up properly. Unless you know what you are doing this requires the system to be drained down.
4. Ideally, connect a new piece of pipe to the existing, having cut the existing a couple of inches above the floor. The new pipe should have a set in it so that it comes straight into the valve. However, done carefully, you could probably get away with what's there.
5. Clean the union fitting attached to the radiator tail to get rid of all corrosion and any old sealant.
6. Clean the compression fitting on the end of the pipe, again getting rid of all corrosion and old sealant. Make sure the olive is in good condition and has not been forced into and narrowed the pipe. If it needs it, replace the olive, which may require shortening the existing pipe and cutting in a new piece with an undamaged end.
7. Smear a little Jet Lube V2 onto the mating surfaces of the union joint. Do the union joint up fairly loosely (say three threads fully engaged).
8. Make the compression joint from the bottom of the valve onto the pipe. Again, do it up loosely. I would use Jet Lube again, although for new fittings it shouldn't be necessary, but given the slight misalignment I'd treat it as belt and braces.
9. Alternately tighten up the union nut and the compression fitting, say a thread at a time, which should pull the pipe over enough if you haven't done 4. above.
10. Refill the system if drained. Test
