How to get dogs to get along?

We recently got a new pup. She is a Dachshund, 3 months... and we had an older dog. He is a Peek-a-poo, and is 7 yrs old. Roxy, our pup seems to want to play with him alot, but Tommy (the older) doesn't want anything to do with her. He growels, and nips at times. How do I get them to get along??