If you are located in south Asian country like India, Bangla Desh or Pakistan or a middle eastern country, the commercial airlines recruit aircraft mechanics on the basis of entry test qualifications. If you pass the entry test and interview, then you are sent to that Aurline ground training school to take courses on Aircraft Airframe or Basic Gas Tturbine or Avionics. Once you pass your ground training courses, you are sent to aircraft hangars to work physically in shifts under supervision if other certified mechanics on the aircrafts. After you complete certain period of time on practical experience, then you become eligible to take Mechanic Examination take by Government depsrtment called Civil Avuation Authority. That will give you ability to work independly on the on line aircraft which is getting prepared to take a flight.
After working as aircraft mechanic for several years , you get eligible to take Civil Avistion Authority's Aircraft Engineer's License exam. If you qualify that exam, then you have the ability to " Sign off " the aircraft as airworthy to fly out on the Technical Log Book.
Aviation is highly regulated field and you need to be a qualified person in order just even to touch the aircraft. You will need to have a good understanding of physics, thermo dynamics and properties of materials to make your way through to the job of Aircraft Engineer. This job carries a lot of responsibility.
If you are located in US, then you will have to join an Aviation School and pay a hefty tuition to go through courses for minimum of 18 months to appear in FAA exam to get your Mechanic Ticket.