Be good if people took readings of their spoke-tension when they first get their wheels on their bike. But I'd take it to a shop, if you don't have a tension-meter handy, and see what they say. Adding a bit more tension to a wheel is a simple matter of knowing where it is now, and where you want it to be. You need a spoke-wrench and the knowledge required to true a wheel. If you don't have the latter, you can take it to your LBS and see what they want for the job. You don't want to tighten the spokes too much or they can round-out or break. So you need to know what's good and safe for your particular wheels.