validations on parameters

hi i have following selectionscreen.

Dropdown option: Tradable to Spare

Spare to Tradable

Movement Type T156-BWART Automatically Populated as 909 Greyed if it is tradable to spare.

Material Description makt-maKTX Automatically Populated Greyed(ref to mara matnr)

how can i get this.

Thanks in advace.