I've fixed tubeless tire punctures with a couple of methods. If the leak is VERY slow I simply pull the tire off, clean the inside of the puncture, and apply and patch. If the leak is a bit fast I've got some plugs that I use. The plug is a rubbery thing that you goop up with cement, insert it into the puncture, then cut of the excess on the outside of the tire. You can find patches and plugs at your local bike shop. As others have mentioned, you can also fill your tire with sealant to stop small punctures as soon as they occur. I tried some stuff from Specialized once but had only mixed results with it so now I run the tires dry and either patch or plug punctures.