If it is important or relevant to the present moment, then use Present Perfect. "I have eaten my lunch." = this is relevant now. I am no longer hungry, and my food won't be left to spoil. The lunch is eaten and because of this, I am now able to work. "I ate my lunch" = done, finished. This is a past event, and mentioned just for your information and doesn't affect what is happening now. The lunch is eaten; let us speak no more if it, and get to work instead. Usage really depends on context and personal taste.

The time frame "just now" is FINISHED, and you must use the past simple. The present perfect is used with unfinished time frames. "Have you had lunch yet?" "yet" is UNFINISHED. You need to think about this for a couple of weeks to really understand the difference here.

I don't blame you for being confused. The present perfect is a confusing tense. If the action is finished you use the past simple. It is only necessary to use the present perfect when the action continues into the present (unfinished time). Action continuing into the present moment. I have been eating this sandwich since this morning....not finished eating it even now I have eaten my lunch for five years in the same room.....and you still do this even now. Action finished at a specific time in the past. I ate my lunch two minutes ago..... You use the past simple when a specific time is mentioned. Of course it’s not so simple. Which is correct when no time is mentioned?. -I ate lunch. -I have eaten lunch. You can choose either. It depends on the context and your preference as Peachey stated. Dialogue: Mother calls to her son’s cell phone while he is in the lunchroom. Son: Hello. Mother: Roger, did you eat your lunch? Son: Hi Mom! Yes, I ate lunch. (by context referring to the recent past using simple present) Mother: Are you sure? Son: Yes, I have eaten my lunch. (switching to Present Perfect to indicate that it is important now…to stress that it actually happened.) Mother: When? (She doesn’t believe him.) Son: I just ate it two minutes ago! Why? (past simple with specific time) Mother: The school superintendent called and told me that you had been throwing your lunch away and buying candy from the machine. Son: I assure you Mother, I have just now eaten my lunch. (Present Perfect again emphasizing the importance of the event now.) Two minutes ago is the same as now in this conversation. Hope this helps. In informal conversations the simple past is often used in America. In formal situations the present perfect is more likely to be used.

Presently, I am simply perfect. :P

Yeah but they're both wrong. LOL.