I am using a Nikon Nikkorrmat FTN camera with 35mm film. Some of the images, once developed, came out fine and some were blurry. Is this down to the developer or my camera. If it is the camera how do I ensure my pictures will come out well?
You used the term "blurry", not "out of focus", which is likely correct. Many incorrectly blame focus when the real problem is moving the camera. A newbee will often just push down on the button with one hand while holding it well away from the button with the same or other hand. That's going to slightly rotate the camera when you take a shot. What you need to do is put your thumb underneath the camera below the button and your forefinger on the button, then squeeze to take the shot.
If all the non-blurry ones are in the same segment, not switching in and out, then lens cleanliness could also be the issue.
The general rule of thumb for sharp images is use a shutter speed that is 1/n where n equals the focal length of the lens used. So with a 35mm lens this would be a shutter speed of 1/50. With practice this can be lowered. Half squeeze the shutter breathe in, hold your breath then exhale as you finish pushing the shutter. I have never mastered any of this in my 30+ years of photography. I carry a tripod everywhere, or settle for wide open apertures.