Nm is Newton-meters, a metric (duh) measure of torque.
You set torque by using a torque wrench. Older kinds look like a long socket wrench with a dial or graduated scale near the handle. When the pointer indicates the desired torque value, stop turning. Newer kinds have a knob at the end of the handle that you use to set a specific value. When you get to the preset torque value, it clicks to let you know. These are the common types.
Torque wrenches are available from most places that sell tools, even some auto parts stores... in the US anyway. I don't know if most torque wrenches these days are calibrated in Nm as well as pounds-feet (lbft)... again, might be a US thing. These are precision devices. Treat them gently, don't drop them, don't use them on stubborn bolts, etc. In general, you probably get what you pay for.
If your tensioner is set properly and the belt is still loose, then you might have a worn tensioner. It could also be that you're not familiar with how tight a belt should feel. But if you say the belt squeals like it's loose, it's a good bet that you need a new tensioner.
Your temperature sounds normal so I wouldn't worry about it. MB temp gauge readings do vary during normal operation unlike other cars where the temp gauge gets to a set point and stays there. If you feel the need to worry, do you know if your fan clutch is working? I'm not really sure how to check without risking your fingers.