It sounds like the bike is pretty new to you. Have you a manual yet? Haynes, Clymer, or preferably factory service? That should be your first step to getting her up to snuff. Next is this forum's tech section. There's guides for bringing home a new bike and that's also where you can find the carb guide Leo linked to. Check out the Buying Guide and General Troubleshooting.
http://www.xs650.com/pages/tech/
Next, the various members' build threads. You'll pick up untold numbers of tips and tricks by reading those. The forum's search feature will help find threads applicable to the issues you may come across.
Re: 5twins' warning. You'll want to get yourself a spare old plug and wire up an alligator clip to it. That way you can clip it to the outside of the motor, grounding it. This will come in handy once it's time to set your mixture screws. You'll be able to safely use the dead cylinder method.
But you have a thorough carb cleaning, shakedown, and proper tuneup to do before you get to the mixture adjustment.