The dilution ratio will depend on your samples. Usually it is 1:5, 1:10, 1:20, 1:50 and 1:100. The sample should fall within the linear range of the standard curve.
You can dilute the sample with the diluent what you use for protein lysis. RIPA particularly.
I usually use 10ul of sample (Cell lysate) with the working solution 200ul of BCA assay kit (solution A and solution B) you can refer to BCA pierce assay kit for further instructions