Ah, the fun hobby of composing. What a complex topic this is. Let's start off with the basics- There are multiple clefs that you can compose in. A clef is a symbol placed upon a staff to indicate the name and pitch of the notes corresponding to its lines and spaces. Since there are many clefs to write in, we will start in one of the basic clefs used for many instruments- the Treble Clef. This clef is used for the higher playing instruments, such as violin, flute, clarinet, etc. The names of the lines and spaces in this clef are as follows-
As you can see, in the musical world, the note names are- A, B, C, D, E, F, G, then back to A (without sharps or flats included, of course.) There is no H, no J, even N, or anything that follows in the alphabet.
The lines that have notes on them above the top line or below the bottom lines, if on a line, are called ledger lines. A ledger line looks like this-
The note below the bottom line of the staff is Middle C. What is Middle C? Well, Middle C is in the middle of the Grand staff, and near the middle of the piano. Ledger Lines can also be above the staff, not just below it. Most of the time notes are written on the regular staff, but not always.
There are many ways of composing. Two ways are melodic and chord composing. Melodic composing is when you make the melody of a piece first, then add the other parts around it, like harmonies and basic chords. The opposite is chord composing. Chord composing is when you write the harmonies or chords before you write the melody and the melody is written to fit the harmony. There's also another way to compose- it's to write both the melody and harmony at the same time. Remember, these aren't the only ways to compose, just some of the major ways. (Also, the names might not be correct, but this is what I think they should be called)
If you can get a melody or a pattern of chords, or even a harmony, it's nearly almost always possible to come up with the other part of the piece. If you write the melody first, then base the chording and the harmony on the melody, depending on what you think would fit. If you write the chording or harmony first, listen to it over and think of a tune you would like to add to the chords, then try it and add it. See how it sounds and if it doesn't fit, try changing a few notes.
Here are the basic notes most people use to compose: Whole notes, half notes, quarter notes, eighth notes, sixteenth notes, and dotted half notes. A dot on a note means that half that note's value is added to the original value. Such as if a dot is placed on a half note, the value moves from 2 beats to 3 beats. Whole notes are 4 beats long, half notes are 2, dotted half notes are 3 beats, quarter notes are 1, eighth notes are half a beat, and sixteenth notes are 1/4 of a beat. This means that 4 sixteenth notes equals 1 beat, like 2 eighth notes equal one beat.
Two half notes in the Treble Clef on middle C
This is a picture of a whole note on the C above Middle C in the Treble Clef.
This is the C major scale starting on the C above Middle C in the Treble Clef.