Why is it that notes "start" with C? In key signatures, for example, C Major is the basis and accidentals are added for all other key signatures. I know that the musical alphabet starts with A and goes to G, so why is C the base note of standard notation and keys? Why isn't A the basis?
Notes do not "start" with C; C major is just the easiest major key to notate in modern notation. The concept of a major key came about long after letters were assigned to the notes. Before there were major (and minor) keys, people used modes, usually just using the notes of the modern white keys and starting and ending in different places. The Ionian mode (which became modern major) was a late addition to the modes.
So it's historical accident that C major is treated as "basic."