Accidentals

In this given example the highlighted note is a F Flat even though there isn't a sign before it. This is because, previously, in the same bar there was an accidental on the F note - which is still effective to the highlighted note. But in the last note, because of the natural symbol, the "Flat" accidental is being cancelled out and thereby the note over here is F Natural 

In this example, the highlighted note is a G natural, this is because this note is present on another bar, therefore, the "flat" accidental on the first note of the piece is not effective on the same note in the next bar.


FINALLY, the most important thing to note is that, the sharp of one note is the same as the flat of the next note.
Basically, D Sharp is the same note as E Flat, or  F sharp is the same note as G flat etc.
This is because, ideally, both of the accidentals- flat or sharp, decrease or increase(respectively) the pitch by a semitone from the original note only.