While it’s technically possible to talk to astronauts and cosmonauts using ham radio, the problem is they don’t get on the radio very often to make random contacts.
With that said, receiving them can be good practice for ultimately talking to them, and it’s fun on its own.
I recommend monitoring ariss.org for upcoming school contacts in your area which happen frequently. You can try and listen to them on the downlink at 145.800 MHz FM.
Another option to stay up to date on the school contacts (also called ARISS contacts) is by subscribing to the amsat-bb mailing list and looking for messages like these: http://www.amsat.org/pipermail/amsat-bb/2016-April/057983.html
You can subscribe here: http://www.amsat.org/mailman/listinfo/amsat-bb