The staff, and middle C as the starting point
Five lines, four spaces and a clef that decides which pitch each position stands for; everything is counted from a single note, middle C.
The staff is a graph: time runs from left to right, and pitch rises from bottom to top. Five lines, four spaces between them, and every line and every space is the place of a note. What the staff does not say by itself is which note is which; that is the clef's job, and it is where you start.
The clef names a line
The clef is the symbol at the beginning of the staff, and it does one thing: it fixes the name of one line. From that line, the others are counted.
The treble clef curls around the second line from the bottom and says that the G above middle C is there. The bass clef has two dots around the fourth line and says that the F below middle C is there. Everything else is counting: line, space, line, space, one letter per position, in the order C, D, E, F, G, A, B.
Notice that the first staff chose the treble clef to write a G, and the second the bass clef to write an F. Each clef is comfortable in one region: the treble for what lies above middle C, the bass for what lies below.
Middle C is the meeting point
Middle C is the C nearest the middle of the piano, and it is the note the two clefs have in common. In the treble clef it sits below the staff, on a short line drawn just for it; in the bass clef it sits above, on a short line too.
Those short lines are called ledger lines, and they are how the staff grows upwards or downwards when the music leaves the five lines. Learn one clef at a time, find middle C in it, and count the rest from there: it is faster than memorising sentences about the lines and spaces, and it works for any clef.
One space per letter
From one position to the next, the note changes letter, always. From a line to the space above is the next letter; from a space to the line above, the next. A scale written on the staff is a staircase with no step skipped:
That is what makes the staff represent letters, not sounds. C sharp and D flat sound alike and occupy different positions: one is on the C line, with a sharp beside it; the other is in the D space, with a flat. The staff distinguishes what the ear does not, and that is why the spelling of a note is not a matter of taste.
Accidentals and the key signature
A sharp or a flat written beside a note holds until the end of the bar. When the whole key uses the same accidentals, they gather at the start of the staff, just after the clef, and hold for the whole piece: that is the key signature, and it saves writing an F sharp on every F in a piece in G major.
The F sharp at the end has no accidental beside it: the signature has already said it.
Reading is counting, until it stops being
At first, reading a note is finding the clef, finding middle C and counting up to it. Before long the frequent positions become immediate recognition, the way a word stops being spelled out. Counting stays available for the rare notes, on the ledger lines, and that is all you need.
What to take from here
The staff is a graph of pitch against time; the clef fixes one line and the rest is counted; middle C is the note the two clefs share and the best place to start counting from. Each position is a letter, not a sound, which is why the staff writes C sharp and D flat in different places. With pitch settled, what remains to read is duration, and that is rhythm's business.