The difference is in the tenses. Had is past perfect, Have is present perfect.
For example: I had decided to go to the theatre before he changed my mind. Then we went to the cinema. There is an action in the past "He Changed My Mind", but before this action in the past there is another action in the further past "I had decided to go to the theatre". So the further past action in the past is past perfect.
John: So, where shall we go this evening. Have you decided yet?
Ann: Yes. I have. I have decided to go to the theatre.
In this example there is one action "have decided", and this action does not have a time about when it was completed and also this action was completed at the closest time to now , which makes it present perfect.