Analyse classic sheet music

What is the best way or method to Analyse classic sheet music? for beginner composer. 
like what thing to look for... Thank you

Use score analysis to answer any questions you have about why you like a piece of music, or why you don't like a piece of music, so you can do the same thing in your own work, or so you can avoid doing the same thing. 

If you really like something that happens in measure 56 of a Mozart piece, try to figure out what happens there, and what has happened in the previous 55 measures that makes that moment feel so appealing to you.  Is it a key change?  What is the old key, what is the new key?  What is the relationship of one to the other?  How is the transition made?

You can also use score analysis to figure out good ways to do something you have never done before musically, or to answer questions about whether or not it is okay to do something.  If you have read a rule in a text book that it is preferred for most chords to be in root position most of the time, go look at some scores and see how often "most of the time" really is.  Compare several different composers, and you'll start to see how the way they treat rules like that gives their work its characteristic sound, and you can decide how you would like to apply the rule in your own work.  

And looking at scores is a good way to find the best ways to write out your own music so that it looks professional and is clear to read.  Are dynamics placed above the notes or below them?  Should you write a dotted half note, or a half note tied to a quarter note?  Should you write an F# or a Gb?  

If you are taking a music course where you are asked to analyze a piece of music, you will want to use the analysis to describe how the music behaves to show the professor that you understand its structure and can communicate about it effectively.  Score analysis is a way of describing in words what is happening musically.  Can you explain a piece of music in detail to someone who has never heard it, without playing it for them, or showing them the score?  

But for yourself, score analysis is a good way to answer your own questions.  How did the composer make me feel that way?  Why don't I like that?  Is it okay if I write something this way, or is there a clearer way to mark it?  Is this tempo playable for the flutes?  What does this range sound like on an oboe?