Hi mathildadawson,
I have two dogs and a cat and they are the bestest of friends. But trust me, it wasn't easy at first. We got Finley when he was a pup and he would always want to play with my cat Alfred and sometimes, my cat would get aggressive with Finley. What's important when introducing a new pet to your cat is to make sure they each have their own personal space where they can go to if it gets too much. Be really patient with your cat and your new pup and always monitor their meetings. Don't forget to reward your cat and puppy when they behave well.
Hope that helped a little :)