Well the first thing you'll want to do is create a callback for the buttons btnTakeHit and btnSell. That way you can add code that executes when you press the button. (in client script)
addEventHandler ( "onClientGUIClick", btnTakeHit , takeAHit, false )
https://wiki.multitheftauto.com/wiki/AddEventHandler
https://wiki.multitheftauto.com/wiki/OnClientGUIClick
function takeAHit( button )
-- Code goes here
-- To determine which drug user used, use the guiRadioButtonGetSelected function
-- [url=https://wiki.multitheftauto.com/wiki/GuiRadioButtonGetSelected]https://wiki.multitheftauto.com/wiki/Gui ... etSelected[/url]
end
Hope this puts you in the right direction!