Look at the bass clef at the beginning staff on your sheet music. You’ll notice that there’s five stacked lines. Reading from the bottom line to the top line in ascending order, the notes are G, B,D, F, and A. For the four spaces that fall in between those lines, the notes are A, C, E, and G.
Use a mnemonic device to remember the note names. Popular ones include “Good Boys Do Fine Always” for the line notes and “All Cows Eat Grass” for the spaced notes. Devise you own if these phrases don’t work. Remember though that notes can always fall above and below the staff as well, and if you know your musical alphabet, you should know that those notes will be E, F, etc. below and B, C, etc. above.
Understand a note's pitch corresponds to placement on the staff. The higher a note is placed on a staff, the higher the pitch. Conversely, the lower the note, the lower the pitch. Study the bass clef on your sheet music and read the note names aloud. Write them in with a pencil if necessary, then, once you’ve gotten the notes down, practice the piece by using a metronome to get the rhythm of the song.