Purchase blank staff paper from a music store or look for it free online. (See resources) You can get sheet music with no markings on it or paper that has clef signs pre-printed. A typical piece of blank sheet music will have about 10 music staffs on it, with each staff consisting of five lines and four spaces. Each of these lines and spaces represents a note name and indicates a pitch.
Purchase a basic book about musical notation. Understanding the basics of music notation isn't difficult, but remembering all the details without help can be a challenge and will take time. Having a reference handy will be a valuable tool as you practice. You can also find a free basic reference guide online. (See resources)
Start with a blank sheet of staff paper and write a bass or treble clef at the very beginning of the first staff on the sheet. Bass and treble clefs have different note names for the lines and staff. The lines for treble are E, G, B, D, F. The spaces for treble are F, A, C, E. The lines for the bass clef are G, B, D, F, A. The spaces for bass clef are A, C, E, G. The clef you write in depends upon the instrument for which you're writing.
Use a key signature reference chart to write your key signature by placing sharp or flat signs on the lines and spaces to indicate the key your song is in. Place a time signature after the key signature. Common time, or 4/4, is an easy time signature to start with. It indicates that you will need to use note values that equal four quarter note beats per bar of music.
Start with a whole note. Draw a circle on one of the lines or spaces. This is a whole note, which takes up one full bar of music. After the whole note, draw a vertical line from the top line of your staff to the bottom. This is a bar line. It separates music into bars. Draw two half notes in your next bar. A half note is an open circle like the whole note, but with a stem attached from the note head. There are two half notes to a bar of music because each half note is worth two beats, equaling the four in your time signature.
Continue your exercise by writing four quarter notes in your next bar of music. A quarter note is worth one beat, or a quarter of the bar. A quarter note looks like a half note, except the note head is colored in. Eighth notes, worth half a beat, look like quarter notes with a tail attached to the top of the stem. If you use more than two eighth notes together, you can attach them by drawing a horizontal bar line across the tops of the stems. Practice writing all of these notes regularly, using different combinations to make up the four beats it takes to fill a bar. Practice enough and notating music will become like a second language.