I'm assuming you're placing this combo box on your worksheet - yes?
There's no programming involved. Create a list of values in a range of cells somewhere. Choose one extra cell to be your 'linked cell'. I suggest that during testing you place these somewhere near your combo box.
If it's a form control, right-click the combo box and select Format Control. Set Input range to your list of values and Cell link to your 'linked cell'. Click out of the combo box.
If it's an ActiveX control, go into Design Mode, right-click the combo box and select Properties. Set ListFillRange to your list of values and LinkedCell to your 'linked cell'. Come out of Design Mode.
You can tell whether it's a form control or ActiveX control by checking the right-click menu: if it has Assign Macro in it, it's a form control; if it has Properties in it, it's an ActiveX control.