I'd imagine this thread will be closed along the lines of there's lots of stuff here, so do some research.
You're really at the very beginning, but to answer your question, in order to get an ATPL, you need at least 500 hours in a multi crew aircraft, and therefore need a job flying in a multi crew environment, typically in an airline.
The way to achieve this is to obtain a commercial pilot licence (CPL) with associated ATPL theory. This is often referred to as a frozen ATPL which unfortunately makes people believe they have something equivalent to an ATPL when in fact they are nowhere near.
Do some research would be my advice.