Pour the tequila into a measuring cup, cut the jalapeno in half (leaved the seeds in), add it to the tequila, cover with plastic wrap, and let it sit for 24 hours.  Take the jalapeno and seeds out of the tequila and mix everything together. Serve on the rocks or prepare it ahead of time and put it in the freezer. In the freezer it will make a sort of margarita sherbet.