It can be a bit difficult to get to the first boss in Portal Knights unless you know what to do, and there are several things you need to complete in order to unlock the Worm Pit.
The first step is to get to the island called Shrieking Sands. This level 4 island contains the first quest, which is given by an NPC called High Cultist Zeebo. The quest requires you to find and give him a C'Thiris Mask, and in return he will give you the Worm Totem Piece.
This Worm Totem Piece acts as a key to unlocking the first boss, which you must travel to Garnet Peaks to use. You will find a Portal on this island where you must place the Worm Totem Piece, and doing this allows you to build the portal that unlocks the Worm Pit, which contains the first boss in Portal Knights.