The first thing to remember when writing classical music is the style of classical music. Classical music is not loud and noisy; it has no words, and is usually played usingnstring instruments. Keep this in mind when composing your music. Try not to write rhythms that have syncopation; it is not used very often when writing classical music.
Next, use simple patterns, along withna repeat process. Have a main theme, and then add variations to it. Then have additional parts of the music that will correspond with the rest of your piece. This is the simplest way of writing classical music, but if has also produced many great results. For example, the song "twinkle twinkle little star" was written by mozart; there are twelve variations in all, and all of them are based on the original piece.
Write in a major key to start it with. This will help your composition greatly. Personally I suggest C Major, simply because it has no sharps and no flats. This will make things easier. Writing in a key that you do not know about can lead to confusion and often will lead you to change keys.
These three simple tips will greatly enhance and improve your skills of writing classical music. Good luck!!!!