The most important thing is to make sure that you're wearing the PlayStation VR headset properly. Put it over your head, and then using the halo around the top, move it around on your nose until the image is totally in focus. Only once you're satisfied with the quality of the image should you tighten the headset in place. Remember, you may also need to move the screen in and out using the button beneath the visor to get the best possible image. Fiddle around with the headset for a few seconds before playing to make sure that you've got it in the right place.
Smudges or fog on the lenses of the PlayStation VR headset can cause blurring, so if you're still experiencing issues, try giving them a wipe using the provided microfiber cloth. You're better off not using water or any kind of cleaning fluid; just wipe the lenses down gently with the cloth, and that should be enough to remove any marks which may be causing blurriness inside the headset.
While the PlayStation VR's default settings should suit the majority of people, you can make adjustments in the Settings menu of the PS4 user interface. Go to Settings > Devices > PlayStation VR, and perhaps consider measuring and adjusting your eye-to-eye distance. In some instances, the correct setting here can relieve any blurriness.
The best way to check PlayStation VR is in focus is by loading a game, and then holding down the PlayStation button to bring up the Quick Menu. You should be able to read the text clearly, and it should look sharp without any blurring. If it's still not clear, consider repeating the above steps until it is. Remember, if you do wear spectacles, you should be able to fit your glasses inside the headset, which could also help with any blurriness that you may be experiencing.