Is this a new problem, or is this a "new to you" camera? If it is a new to you camera did you shoot a test roll using all the apertures and at least one infinity shot at the smallest aperture? If you made that test shot and it is out of focus here is a list of what could be wrong.
You were shaking, all the labs messed up, there is something hiding between your mirror and your shutter, there is something wrong with the film guides (pressure plate or lands), your lens and screen are both mounted wrong, you were "floating" a lens and are not that steady, or your camera is done.
Before you load more film check the film pressure plate, and machined flats, the pressure plate on the door should be springy, and the machined surfaces clean. Remove the lens or front cover, and with the rear door open set the camera in B and open the shutter and look through to be sure there is nothing in there. Check the screen, I don't think you can remove the screen on an FM2 but check it anyway if you can remove it Nikon has instructions, follow those to remove and replace it. Be sure your lens is properly mounted to the camera, not loose or shaky (no lens "floating" either, this is a test roll). Load film, and shoot focused on something at infinity at all apertures, then something close (less than 10 feet) at all apertures, make all of these shots with the camera on a hard flat surface or tripod and use the timer so you are not touching the camera when it takes the photos. If after all that you are still get back out of focus photos, it is either your lab, or a problem with your camera that you can not easily fix at home.
Good luck.
