was saying / have said / did say

When I am having a conversation with someone and want to make clear that I for example agree with her opinion because i am thinking the same, can I say: "Yeah, this is what I was saying / talking about" or "Yeah, I have just said that" ?

But when I talk to a child I try to remember a child I have to say: "What did i tell you about that" when I told to him/ her some time ago, But when I've jus told it, I can use: "What have I just said", can't I?

Thanks!