I would advise you take some time to learn how to read sheet music. Teoria has a set of beginner tutorials that will really help you to understand how music notation works, and how to understand what all the symbols and notes mean. That said, on to the question.
In the key signature(the 2 b signs on the left) you have a Bb and an Eb. this means that every B and E noted will actually be Bb and Eb respectively.
That's it for key signatures, that applies across all bars. However, you also have accidentals which only take affect for the bar which they occur on. the sign on that note in the first bar is a natural sign. it means ignore all sharpening and flattening that is being applied to this note for the rest of this bar.
As I mentioned before, accidentals only apply to the bar they are on. that's why you have a Bb in bar 2 and not a B natural.
In bar 3, the C is sharpened to a C# by the # sign. as I said before, all accidentals only take effect to the end of the bar, so it will not sharpen the C in the next bar.
Why the C is natural
Okay, now on to the bar with the C in it. As I said at the start, the key signature affects all B and E notes, so the C note will not be affected. The # sign from the bar before stopped taking effect before this bar, so there are no alterations to be made to C notes appearing on this bar. this means that the note marked on bar 4 is an unsharpened, unflattened C, also called C natural.
I've been as thorough as I can, I hope this helps clear up any concerns :)