Visit Noteflight.com and sign up for an account. It's free; all you need to do is create a username and password. As soon as you've done this, log in to your Noteflight account to get started.
Click the "New Score" option on the option bar at the top of the work area. Choose the privacy level you want for your sheet music. You can select from "Private" or "Shared," depending upon whether you want to allow others to view and comment on your online music sheets. Once you choose your option, Noteflight presents you with a blank sheet music in the key of C, 4/4 time.
Begin writing your song. If you like the key and time signature, you can begin placing notes on the staff by selecting the type of note from the floating tool palette, hovering over the position you want to place the note on the staff and left-clicking your mouse to place the note. To change the key signature or time signature, point to it with your mouse, double click, then select the new elements from the floating palette. Compose your entire song with these steps.
Listen to your song before you print it. Do this by going to the "Play" menu and selecting the "Play from Start" option. You can also listen to your song as you compose using this menu. To add lyrics to your composition, go to the "Score" menu, select "Text" and choose "Lyrics." Now you can type your song lyrics right onto your score so they are positioned under the proper notes.
Print your sheet music using regular computer paper. Noteflight prints a professional score for you. You can also save your score as a .wav or midi music file by going to "File" and selecting "Export." Use the "Score" menu and select "Parts" to assign instrument sounds to the parts of your score.