Make sure a clear ultraviolet filter is not screwed on the end of the lens. Often dust will be trapped between the filter and the lens. Look at the side of the filter (if there) and you should see markings indicating size ( example 72mm), brand, and type. These filters can become very tight, but never use any tool to try to remove. Grip with cloth or rubber tipped gloves to remove.
if no filter is screwed on the lens, and you see dust inside, that is a job for a camera repair specialist.