I'm afraid that you'd rather read some PL/SQL documentation, just to get the general idea of how a PL/SQL block looks like (declaration section, use of DATE datatype (pay attention to the TO_DATE function), ...).
Then read your Forms manual (if you don't have any, Forms Online Help system will be OK). In there, you'll need the SET_ITEM_PROPERTY built-in.
It appears that WHEN-NEW-FORM-INSTANCE is a better choice than WHEN-BUTTON-PRESSED trigger.
Finally, when you've done with all of that, you will be capable of producing a much better code than the one you have provided now (because this almost isn't worth reading).