When your car was designed, each and every bolt was tightened to a specific point, called torque. Torque can be measured in lbs/ft, lbs/in, or NM. Most spec sheets will offer lbs/ft, or "foot-pounds" as they call it in the industry.
As you tighten a nut or bolt, your torque wrench will tell you when to stop, either by indicating the tightness on a dial or by giving you a nice click, click sound when it's right. It's a must-have tool if you plan to do any mid-level repairs. There are also needle-type torque wrenches that have a visual gauge and a needle, but they are less precise and a little harder to deal with in my opinion.
Buy a torque wrench.
* You can purchase this tool online through one of About.com's trusted online retailers.