Be specific which element you want to detect. As different elements have different methods. Also the reagents and volumes will differ as the element changes. Plotting the calibration curve is very easy once you know the element, you will need to prepare the known standard concentration and then plot  Abs. vs. Conc. and you are there. 

specifically for cadmium detection. How we can prepare solutions. and wot reagent can be used ?

Yoy may try to extract Cd from complex solutions by the Amberlite method as iodide

 at pH lower than 3,0 .Wash  with KI  1,5% (with 0,2 ml H2SO4)- Re- trasnsfer to waterr with Na2CO3 10%-  Then, colorimetry with dithizone.