Hi,
a) you can have internal or external clock sources. Some microcontrollers can slo down (divide) the clock, some microcontrollers have an internal PLL to create higher or lower clock.
a, b, c) To start with it and to get a solid idea of what a microcontroller can do and how it works....
* read some datasheets. You don´t need to understand each detail at once, just to get an overview...
* then buy a cheap microcontroler module and a programmer. Very common and modern are PICs and AVRs (I prefer AVR, others prefer PICs)
* it can be a low end one... no need to go into details at the beginning.
* then read/view tutorials on how to start. Switch on/of a LED by key press. Blinking LED.....a lot of inbetween - use ADCs to measure voltage....
Good luck
Klaus