Sign up for a Noteflight account. Noteflight is a free, web-based music composition software.
Log in to your Noteflight account. Click the "New Score" command at the top of the page, then choose whether you want the score to be private or shared. The default blank sheet music is a grand staff piano sheet with a bass and treble clef in the key of C with a 4/4 time signature. Click "Edit Title" at the top of the page and type in the name of your song, click on "Composer" and type your name.
Set your sheet music up according to the instrumental you want to write. Keep the default if you want to start with a piano part. Add notes to your score by clicking the blank music staff. A note will appear and you can drag it to the location on the staff where you want to place it. A floating palette stays in view so you can select the notes and symbols you need to compose your music.
Add musical instrument parts to your score by hovering over the score. You will see numbers along the top of the score. Select the number matching the location where you want to add a new instrument part. Go to the "Score" menu, select "Instruments" and select either add "above or below" the staff. Choose your new instrument from the pop-up list that appears and a new staff for that part will appear.
Finish composing your piece. Noteflight lets you listen to your composition as you go, with each of the instrument parts realistically rendered. When you are satisfied with your composition, print a professional sheet music score and use the "File" menu and the "Export" command to save your composition as either a wav or midi file.