The first set of frequencies belongs to the on-board amateur radio station. Voice contacts are primarily reserved for schools, however, if a crew member has some downtime they’re free to go on-air and make as many contacts as they want. As long as there isn’t any electrical problems the packet modem will also remain on and in ‘store and forward’ mode. Allowing personal messages to be sent to crew members and other hams around the world. The SSTV frequency is rarely used, mostly during special events such as Field Day.

145.8000 FM Region 2 Voice Downlink
144.4900 FM Region 2 Voice Uplink
145.8250 FM Packet | APRS
437.5500 FM Packet
145.8000 FM SSTV

The second set of frequencies are for commercial operations such as EVAs and the docking of resupply ships. On VHF-1 you will hear Cosmonauts communicating with one of several Mission Control Centers around the world. VHF-2 is said to be used by Astronauts though Russian comms have also been logged on this frequency. On 121.750 and 121.125 EVA ‘spacewalk’ activity has been noted with the callsign ‘Soyuz’ being heard on EVA-1. EVA-1 is commonly logged by radio operators in SoCal as its also used by ground crews at LAX. Do note the ISS operates on Moscow Time so you may find the crew more active late at night.

143.6250 FM VHF-1
130.1670 FM VHF-2
121.7500 FM EVA-1
121.1250 FM EVA-2

If you’re worried about needing a big set up to monitor the ISS don’t worry, its just like monitoring AO-27, SO-50 or any other FM satellite. All you’ll need is a scanner, general coverage communications receiver or a 2-meter radio with extended receive which pretty much all radios made now a days have. As for an antenna, you will want to use something with gain like the Diamond RH-77/SRH-77 or even a beam antenna like the Arrow Antenna or Elk Antenna. They are fairly low cost and extremely portable.