How do I stop my dog barking?

I took in a ten month old lab x crollie from a rescue two weeks ago who
barks whenever she sees anyone in the park, or if she sees the postman
through the window etc

I also have a four year old lurcher who is very calm and placid but when she
hears the lab barking it then winds her up and she joins in.

Does anyone have any advice on how I can stop this?

TIA

Purple