If the instruction is 8 bit it can do 256 different things like NOT, OR, AND, etc. It the instruction is 16 bit it can do 64000.
It is typical for there to be some bits to say what instruction and some to say where.
example: Bits 7,6,5,4 give 16 operations like like AND. Bits 3,2,1,0 give a location. A, B, C, D,......
Instruction AND R,B OR R,C <instruction><Address>
With a 8 bit instruction you do not have much room for address so usually you have a operation, address and the results must go to the "register".
If you chose a longer instruction then you can have 8 bits for instruction and <address1><address2>. This way you can have <AND><D><E>.
I think you should get the data sheet for a 8051 or a PIC16F64 and look at their instructions. Also look at memory and registers. Some parts like 6805 have memory mapped registers while most have registers that are different than memory. The AVR parts are also good to look at. Do not start out looking at a ARM. They are very complicated.