Start out car camping. My progression was:
1. Avid day hiker (10-16 mile hikes every weekend, 5-8k ft gain)
2. Car camping in Ojai (near Los Angeles). Drive in, setup your tent, chill, sleep, leave next morning.
3. Backpacking around LA on the trails from step 1
4. Thru-hiked PCT
5. Mountaineering / alpine climbing
Lots of people do the PCT with little hiking experience and are fine though. It's just walking, and there are tons of people to help you.