Man Mo temple can be reached by taking the MTR to the "Sheung Wan" stop, then heading about 6 blocks uphill. As you go uphill, the streets no longer follow a perfect grid, so you may need to navigate around some buildings.
At this temple, locals come and worship the Gods Man Tai and Mo Tai. Students come here to ask for good grades. As a tourist, you can buy some incense and light it up, if you would like.