Let's start by correcting your question. Two things: Remember that the verb or auxiliary verb has to go before the subject in a question, and that 'must' is a modal verb so it doesn't need 'to' after it. And a third thing - the pronoun 'I' and languages always have capital letters. So your question should be:
When must I use the present perfect in English?
These are the main uses of the present perfect:
1. For a situation which began in the past and is still true now e.g. 'I've lived in Italy all my life'.
2. For an unfinished period of time e.g. 'I haven't eaten anything today.'
3. For an unspecified period of time e.g. 'I've seen that movie three times.'
4. For anything which has an impact on the present situation e.g. 'I've lost my mobile phone.'
Finally, two more tips:
- The present perfect is a present tense. It always tells you something about the situation now.
- The present perfect is NOT the same as the Italian passato prossimo. Don't believe anyone who tells you that it is.