You have to find the summonsing altar. It will be a stone table and a stone depiction of a deer. Usually it's not too far from the beginning point.

You need to build a bed in a sheltered building (needs to have a roof over it), then chain it to set that location as your new sudden point.

Also, you can look up the Valheim World map generator, enter your seed (click any options for bosses, etc and hit search.