Getting a new dog

Hello all, I have a 6 year old female Jack Russell Terrier. She is energetic and sweet , well socialized however sometimes overly protective especially when it comes to me. Recently I started thinking of getting a new dog as I feel it would be good for Poppy to have a companion. I am not sure what dog would suit or even how to begin with deciding on breed. Any advice is more than welcome. Thank you. Jasmina