When you find schematics - do get the datasheets for the components.

The datasheet for the MAX232 will explain how the chip works and how you use the chip. That allows you to understand that part of the schematic.

The datasheet for the processor (more than just a "datasheet" since there are huge amounts of documentation for a processor) will document all individual processor pins. That will tell you about the RST signal and what to do with it.

Note that this schema isn't using a crystal but a crystal oscillator module. It costs more than the crystal but contains a number of extra components and produces a TTL-level clock signal that can be directly fed to the processor without the need for extra load capacitors. Note that the power consumption can sometimes be quite high for crystal oscillators, since they are more optimized for being stable while running with very high load (many listeners or longer signal trace lenghts).

The datasheet for the regulator will tell you if the circuit is limited to +9V supply voltage, or if you may use 12V, or maybe 15V... And it will tell you if you have enough current for just the processor or if you can also power other chips or use the I/O signals on the processor to sink current for a number of LED or similar.

By the way - the original circuit showed a battery symbol - was your intention to have a battery-driven circuit or was that just incidental that you managed to copy a circuit with a 6V battery?