There are three ways, however, I am going to assume you are a new player as the other two ways are for medium to high levels who definitely know the answer to this at that level. The method you should do is to simply go to the red exclamation mark on your map and look for the candle seller near it. He sells a rope to go down into the swamp. Just use the rope on the hole, and you can now go inside the swamp. Do not forget a light source and a tinderbox if you go down there! I recommend a lantern to avoid the explosions of the swamp gases.