Step 1: Fix the basic identity of each.
RBI is the single central bank of the country, while commercial banks like SBI, HDFC or ICICI are the many banks that deal directly with ordinary customers.
Step 2: Compare their main job.
RBI's job is to control the money supply, set policy rates and keep the financial system stable for the whole economy, while a commercial bank's job is to take deposits and give loans to individuals and businesses to earn profit.
Step 3: Compare their powers.
Only RBI can print and issue currency notes and set tools like the repo rate and CRR, commercial banks have no such power, they simply operate within whatever rules RBI sets for them.
Step 4: Compare their relationship with the public.
RBI mostly deals with the government and with banks rather than the public directly, whereas commercial banks open accounts, give loans and offer everyday banking services straight to people, which is the key difference between the two.