That's kind of a relative question, because it's entirely dependent upon what you're trying to do. If you have written or recorded a macro and you want it to be triggered when a button is clicked, then copy that code and put in place of 'Put your code here.
I.E. if you're trying to bring up a user form with the checkboxes that you mentioned in your previous post, you'd do this:
Private Sub CommandButton1_Click()
Note that the code for Control Toolbox button's goes in the sheet's module, whereas a macro assigned to a Forms button needs to be in a general module.
I can e-mail you a workbook with various button examples if you'd like.