You can apply for jee mains exam when you  have passed intermediate or equivalent course of 10+3 which includes your completion of diploma. The government now considers diploma completion equivalent to 12th standard completion. Only then you can apply for jee mains and once you clear jee main, you will have to write jee advanced, only then you can get a seat in IIT, if you only complete jee mains, there is no eligibility even to get a seat in NIT or CFTI(central funded technical institute), after sucessful completion of jee advanced, you will get a seat in IIT in the 1st year, not second year, as IIT don't consider the method of lateral entries like ordinary engineering colleges.