Most of the universities run their own clubs, but you can always check these out. The biggest and most broadly recognized is the ESN or the International Exchange Erasmus Student Network. They can basically help you out with anything, they organize orientation weeks, set up trips to go all over the country and
host other social activities where you can always meet some new people. There is the International Students Club of Czech Technical University in Prague, which functions in the very same way. Another one is specifically tailored for the students of Czech University of Life Sciences – you can find it here. Apart from these you may try AIMS or the Association of International Medical Students, or the Trimed for all the students of the Third faculty of Medicine.