Ooh. This is good. You can watch this video and I don't have to write anything about sharps and flats. It's all here, and very clearly explained.
I'll keep a look-out for good examples in our music as we sing.
Here's a simple visual of how the accidentals work, with flats.
Here, there's a key signature with F# and C#. Notice the C in the second bar is natural. The one right next to it will be natural too. The C an octave down, in bar 3 is also marked natural, the one in the same bar will also be natural. The first C in bar 4 is sharp, because it's a new bar and the key signature comes back into effect. The following C is marked natural to cancel that sharp.