"I had..." describes a past situation. This also implies you don't have it now: the action is done and finished. "I've had..." explains the present situation. This is basically how present perfect works: a past event is connected to the present. Here are some examples: "I've had (=eaten) breakfast." = I am not hungry. "I've had this shirt for only six months." = I have a six-month-old shirt. "I've had the pleasure of his acquaintance" = I have a good opinion of him, because we met in the past.

'had' can be used a few ways: Had can be a past tense verb that means 'own' or 'hold'. That is, the thing it refers to is in your possession. So you could use "I had" to say that "I had a book." This means that you held the book, but no longer hold it. This can be used for things that are not physical -- you can say "I had things to do." to say that you were occupied with some other task. You could also say "I had bread." To say you ate bread. There's a tense called pluperfect that uses had to show time, but I am out of examples.

Learn to use the present correctly before learning the past tense!