My wife already has a two year old baby and is expecting our first child soon. how do I make sure that she is fair to my child?

I think this question applies more to you than her because biologically she is the mother of both the kids but you are not the biological father of both. So, in that case she must be the one who would be worrying about this thing that whether you will love her child from the previous marriage the same way as you would love the one who is coming. She is mother to both the kids so she would love them equally like any other mother with two kids does.