One of your first stops should be Tourism Office of San Marino. Here, you'll be able to orient yourself and get some great ideas before you start exploring.
You'll have sightseeing choices galore in San Marino. Tourism Office of San Marino is a good place to begin here, but there are many other worthwhile sights to enjoy. Palazzo Pubblico, Guaita Tower, and Liberty Square are not far from the tourist information center and and will cap off your day nicely.