Need basic info about Programming microprocessor and its principle of operation
Basically you read the processor's databook, which will tell you how it starts executing, and design the board so that the ROM is mapped to the appropriate address.

For example the Z80 will assume whatever it finds at address 0000 is a valid instruction and execute it. So, when resetting a Z80 after power on, you should map the ROM to address 0 and put your startup code there. 

Later, you can map the ROM to another address or even select it out of the memory map altogether, but before you do that, typically you would load an actual OS like CP/M from floppy disk, into RAM, map the RAM to address 0, and either warm reset or simply jump to 0.

Other CPUs may start up in different ways, the 8086 starts at address FFFF0, but all have in common that their documentation tells you where this first instruction or Reset Vector is located.