I have a leak coming from behind the water shutoff valve under my bathroom sink. The pipe coming from the wall is plastic. I don't know how what it is that attaches the pipe to the valve. I tried pulling it off but it didn't work. I tried tightening it put that didn't work either. If anyone can help me I would really appreciate it.

It is the pipe in the wall leaking? Or a fitting is leaking? If it is plastic it is either a compresion fitting that needs tightened or if it is pex the it is probably a crimp fiting and can not be tightened. Easyest thing to do if does not stop by tighting it is just to get a new shutoff.

You will probably have to cut out the wall to see if it is leaking there sounds like its coming from behind the wall