You must choose an existing creature type, such as Vampire or Cat. Card types such as "artifact" can't be chosen.

Once Kindred Dominance begins to resolve, no players may take other actions until it's done. Notably, players can't try to save their creatures after you've chosen a creature type.

You can't choose multiple creature types, such as "Cat Warrior." A Cat Warrior is both a Cat and a Warrior. It's affected by anything that affects either type and unaffected by things that affect non-Cat or non-Warrior creatures.