Present perfect vs. Past perfect for finished actions [duplicate]

When is the perfect tense used? All examples illustrating perfect are always answer to the same question. So there is no way when I can simply said:

I have written a note
or
I had written a note

Is that right? Instead I need to say:

I wrote a note

But if someone asked me: Did you finish a note? then I need to use perfect:

Yes, I just have written a note
or
Yes, I had written a note yesterday

Am I right?

In TEFL we usually teach four areas where Present Perfect is used:

• events in an unspecified past - this has been covered by Guffa
• events with some sort of relevance to the present - psmears has dealt with that one

if I could just add:

• events in a current unfinished time period, that's to say: today, this morning (depending on time of day), this week, this year etc.
• use with certain words, such as: ever, never, since, for (but for can also be used with Past Simple) eg:

I have lived in Warsaw for ten years / since 2000 (and am still here)

Before that I lived in London for eight years

In addition to what Guffa has written in his answer, the (present) perfect usually expresses a current state resulting from a past action - either an accomplishment, or something that has consequences. For example:

• She has arrived (so she is now here)
• I have come (and am still here)
• I have studied a lot (so I know many things)
• I have spent a lot of time in the gym (so I am fit)
• I have finished my homework (so I can watch television)
• I have bought a car (and so I now have one)
• I have eaten (so I don't want more food)

The past perfect (/pluperfect) does the same, but talking about a state in the past resulting from a prior action:

• I had studied a lot (so I knew many things).

No, you can use either form in either case, depending on what you want to express.

Use past tense when you mean a specific point in time in the past:

I wrote a note yesterday morning.

Even if the specific point in time isn't specified, the form still implies that there exists a known point in time where it occurred:

I wrote a note.

Use present perfect for an unknown or undisclosed point in time in the past:

I have written a note.

Each works either on their own or as an answer to a question.

If you want to express a specific point in time, you can't use present perfect:

I have written a note last monday. [Wrong]

You have to use past tense for that:

I wrote a note last monday.