Parking at all SMART lots is pay, and you must use the Parkmobile app to do so. If you go on a Sunday, on-street parking may be free (I understand it is in San Rafael). Try Googling "parking" for any city and see what information you can find. The Marin Civic Center SMART station has no parking lot, but there should be plenty of free on-street parking on the street that parallels the track (don't remember the name, but it goes past a hotel to a residential area).

You must use a Clipper Card to pay the SMART fare. This card is used on many other Bay Area transit operations (BART, Caltrain, Muni, etc.). If you don't already have one, you can purchase one from machines located at each SMART station. You tap it on a reader before you board (and after you get off), but the conductor also checks them onboard.

Riding SMART is great fun, more for where it is than what it is. Who would have thought that after 60+ years, you could ride through Marin and Sonoma on the NWP? Since trains are only 2-3 cars, commute times (and sometimes weekends) can be very crowded. Best bet is to ride mid-day.