I am a professional, so I will tell you how to fix your mouse, if it is old.. 8-)
Turn it over. Do you see a little ball? if not , you must buy a new mouse. If so, twist the little ring and pop it out. Your will see the small hard rubber ball. Turn the mouse over, and the ball will fall out, only the ring was holding it in. Now turn the mouse back over, and look carefully. There should be 2 little rollers, one in front of the ball, and one off to the side. They are probably crusted with accumulated crud. Take a small sharp knife and carefully scrape the crud off the rollers, they should be smooth. Replace teh ball and the retaining ring, and your mouse should work much better.