To begin Geralt's journey through The Witcher 3: Blood and Wine, head to a contract notices board in Velen. Once you receive the contract, head to Holloway to meet the knights from Toussaint.
The knights are there to bring you a message, but first you'll have to deal with a few common bandits who have been terrorizing the village. After a brief battle with several, the knights will read their message from Duchess Anna Henrietta. A beast with many forms has been terrorizing their land. They ask Geralt to accept the contract and kill the beast.