The alto clef places middle C on the middle line of the stave. The alto clef is most often used for viola music and sometimes alto trombone. This is what the alto clef looks like:
There are many different mnemonics that can be used to remember the notes on the alto clef (reading from the bottom line up).
You could use this mnemonic to remember the notes on the lines:
You could use this mnemonic to remember the notes in the spaces:
If you need to write notes above or below the stave, you use ledger lines and they look like this:
To avoid writing notes with lots of ledger lines you can change clef which can make it easier for musicians to read your music.
If your notes are getting too low you can switch to bass clef:
If your notes are getting too high you can switch to treble clef: