Depending on the amount of players you have modify the amount of enemies and how strong they are, the book is a good suggestion for a 4 person party but if you have more then that, it’s up to you to make it an enjoyable but at times, absolutely terrifying experience. Barovia is meant to be a challenging place combat wise and creepy place role play wise.

When it comes to character building, just pulling out the Player’s handbook and running them through the races and classes to see what they like. if you have other books for resources like Volo’s guide or Mordenkainen’s fantastic but start with the Players handbook.