If you have trouble remembering the notes of the six strings on your guitar, you may find it helpful to think of your fret board as the keyboard of a piano. Find middle C on your guitar and then the 2nd E lower than this note: this is the low E on your guitar, which is the sixth and lowest string. The next note going up on a guitar is the same as that on a piano, A. Moving along; we have D which is the same as the next D on a piano's keyboards and then G, B and high E. The high E on a guitar corresponds to the E which is above middle C on a piano.
However, if you're not familiar with the piano, then the strings are like this from highest to lowest note: high E, B, G, D, A and low E.
In musical notation, these notes are written on the five-line musical staff with a treble clef. Every space and every line of the staff stands for a note of the musical scale which runs from A to G. From bottom to top, the lines on the staff are E, G, B, D, F, which people usually memorize with the mnemonic "Every Good Boy Does Fine".
The spaces from bottom to top are even easier to remember - they are F, A, C, E which is easy to remember since it spells a common word. The same is true for the piano when looking at sheet music; though of course piano music features two staff, one with the treble clef, the other with the bass clef.
When written out in musical notation, notes are represented by different shapes which denote how long to hold the mote rather than by the letter (since this is represented by its position on the staff). A hollow oval is a whole note; a hollow oval with a stem is a half note. A solid oval with a stem is a quarter note.
Flags on these stems tell you if these are eighth notes (one flag), sixteenth notes (two flags) and so forth. There are also rests in various lengths which have their own distinct shapes so that the reader knows when to play as well as where to pause.
When notes which are too high or low to fit on the staff, then lines and spaces may be added above or below the staff as needed. These extra lines are known as letter lines; the notes which they represent are an octave above or below their counterparts which fit into the staff.