Which is correct – “I can have an apple?” or “Can I have an apple?”
The order (sequence) of words in an English sentence is often different from the word order of the same sentence in other languages.  The word order in a question is even a little more difficult.

This picture may help you.  It’s called the “Question Hand”.  To make a Yes/No question, start at the pointer finger.  The “X word” can be an auxiliary (helping) verb or the only verb in the question.
Examples: Did (x word) he (subject) eat (main verb) dinner with you?  Is she coming home?  Can I have an apple?  Are you okay?

To make other informational (open-ended) questions, start with the thumb.
Examples: Where (question word) do (x word) you (subject) live (main verb)?
What time did you get home?

Try to put the words in these questions in the correct order:

And here is some more explanation, plus many more practice exercises.