Agree with Dru in that interval reading is the way to go, but takes years and years to master, and then more years.
It just so happens I've been studying Lean On Me
, every now and then myself, and the intro, which is then used in the beginning of the melody again, is quite simple.
It starts with a C chord, then Dm(inor) Em and F, up the keyboard and then down again. You're using first inversion chords, meaning you play the root at the bottom, for example in a C chord, EGC instead of CEG. The intro finishes off with a G chord, followed by a C hord.
This version is a bit different than the one I've been following, in that the intro part skips the fifth of the chords, just plays EC instead of EGC in the treble, and the same with the other chords.
About recognizing the notes in the bass, well it's a C below middle C in the first three measures. To make it not too easy for you, I'll give you some hints to start you off in the rest of the notes.
Since the piece is in the key of C major, which has no sharps or flats, only the white notes, you can make use of the rule that if you have a note on a space, and the next note you have to play is on the line above, you can rest asure it's a D, the next note in the C major scale. If you were supposed to go down from the C, to the B below, it would lie on the line below the C.
What I've just described is called intervals. Of course an interval (going from one note to another) can be larger, but going from a note on a space to the next line immediately above it, or from a line to the next space, is an interval called a second. Like for example C to D (notes.) Going from a space to a space is a third, from a space to the second line above it a fourth, and so on.
This way you can count your way from every note on the sheet music. It's even more complicated than I've just described, but welcome to the world of music theory!