How to make my Nikon D70 pictures less grainy?

The tittle says it all

Less grainy? I'd say your ISO is set too high. Read your owner's manual for details and set it to 100.

If the camera is in auto mode, you must be shooting in low light. The camera is using a high ISO setting to compensate. Move to a better lighted area to take pictures. Take note that low light to you may as well be pitch black darkness to the camera so learn to adjust.