A torque wrench measures how much "twist" is being applied to the bolt to ensure it's not over- or under-tightened. In most torque wrenches, a torque value is set into the wrench and then the wrench will indicate the value has been reached. With a beam-type wrench, there is no presetting; you just watch the gauge until the pointer hits the right value. Other wrenches will click, break over, or in some other way indicate the preset torque is being applied.
Unlock the adjustment mechanism. This will differ, depending on your wrench. Some torque wrenches require that you remove the end cap from the handle; others only require you to pull the end cap to release the lock. Still others have a lever in the end cap that needs to be revealed to set the wrench. Check your wrench and your owner's manual to see which one your wrench requires.
Set the torque. Most of the time, this is done by twisting the handle or rotating the lever that was revealed. Clockwise rotation will usually raise the torque setting; counter-clockwise rotation will usually lower it. Sometimes, a hex key needs to be inserted and turned to set the torque. If you're using a break-over style wrench, align the wrench before setting the torque.
Lock the adjustment mechanism. This may mean returning the end cap, pushing it back in or replacing the lever.
Preset torque wrenches are designed so they cannot be adjusted. They are good for use in places where the same torque is needed repeatedly, including assembly lines.
Never set your torque wrench outside of its designed limits.