The fortress Shlisselburg is located 67 km from S-Petersburg city on the tiny island in the mouth of Neva river.
One way will be on the electric train to Nevskaja Dubrovka from Finlandskij railway station in S-Petersburg. From S-Petersburg to "Petrokrepost" station is a good one and half hours journey. Shlisselburg city is located on other side of river. From the platform you should go in the direction of Neva river to the pier near to 10 min. The little steamship goes to the fortress island from here. Departure of steamship is once an hour. Sailing takes near to 10 min and costs 10 rubles.
Other and more reliable way is to take a bus to Shlisselburg city from subway station "Dybenko" in St.Petersburg and than take the steamship on the quay near to Peter the Great monument. You can hire a private boat to the fortress too.
It will be wise to buy a beer in S-Petersburg because Shlisselburg's beer is more expensive.