Understanding key signatures

[jennd]

I'm a beginner and I'll be taking lessons come fall. I'm only concerned with learning to play this piece right now (https://musescore.com/user/113382/scores/125731#). I know I ought to wait to learn so I can play the right way, but I've got over a month and I'm not waiting. Please excuse my newbie ignorance.

Up until the sharp appears on the second page in the link, the piece has been all flats. So does the sharp mean up a semitone from a normal C key (C sharp), or does it mean up a semitone from C flat (just C)? Also, sharps, flats, and naturals are all accidentals and only apply to the note to their right and through the rest of the measure, right? And then I would start playing the key signature again? For example, the sharp on page 2 would only apply to that note next to it, and then the staff beneath that would go back to flats, yes? Last question: Do naturals mean go back to flats or play the regular, white keys?

[dogperson]

The main key signature has one flat (B), so all notes in the piece are natural keys (white) unless otherwise marked, except for the B.  B's are always flat unless the markings give you an exception for a limited period of time.

 The markings I see on page two are a natural sign in front of the key "B"..  This tells you this B is not flatted, like the remainder of the B's in the piece, but a "natural" B (white key)   The change from the normal B Flat to B natural is for that measure only.  The next measure goes back to all B's being flat again. 

The other thing you need to know that these exception markings onlY apply to notes on exactly that same line.  So if a B is marked as natural on one line, and there is another B on a different line in the measure, the "B" on the different line is played like the B flat

It's great to see you are so excited! 

[jennd]

dogperson--
Thank you! I'd thought the key signature applies to every note! You saved me a whole lot of frustration.

[keypeg]

Your piece is in the key of D minor.  The key signature with one flat can be for F major or D minor.  Here is a bit of theory for you.

F major:  F G A Bb C D E F
D natural minor:  D E F G A Bb C D

Very often in a minor key, the 7th note is raised by a semitone.  This will give you:
D harmonic minor:  D E F G A Bb C# D

You can also have a melodic minor where the 6th note is raised:
D melodic minor: D E F G A B(natural) C# D

Your music has a B natural in places, and a C# in places.  This is why.

[dogperson]

dogperson--
Thank you! I'd thought the key signature applies to every note! You saved me a whole lot of frustration.

The key signature does apply to all notes in the piece..  but in this case, the key signature has one flat (B), so it tells you every note in the piece is a natural except for the "B's".   If you had a key signature with one sharp (F), every note would be a natural except "Fs"... they would be sharps.  

The key signature outlines what to do everytime you see a particular note in the piece, unless you receive other instructions.  Without the key signature at the beginning, this piece would need a flat sign in front of each and every "B".... what a mess!