What are the major civilian plane crashes in history? What was the first major one? How have they changed the way we fly?

I can't speak to the first major one, but crashes and their investigations have dramatically changed the way airliners are built and flown. A couple of key examples off the top of my head.

Tenerife in 1977 is still the deadliest single accident. It was a foggy day, lots of planes had diverted to Tenerife, and two planes collided with one another as one (a Pan Am 747) was taxiing down the runway and at the other end another one (KLM 747) started its takeoff roll. The biggest contributing factor for the accident was found to be the captain of the KLM flight ignoring his copilot who believed there was traffic on the runway, and starting his takeoff roll.

This resulted in the widespread adoption of Crew (or Cockpit) Resource Management, or CRM. It makes Captain and First Officer, as well as cabin crew, almost equals as far as how they pass information and interact with one another.

The Delta 191 crash at Dallas/Fort Worth in 1985 was another huge incident that changed aviation safety. The Delta flight, a Lockheed L-1011, was descending towards the airport when it flew through a "microburst," which is is essentially a fierce, powerful, but short lived downdraft. At the time, these phenomena were not understood, and were not detectable on radar. The plane experienced such a fierce downdraft that even at full climb thrust and attitude, it crashed short of the field.

This crash brought about advances in ground based weather radar, typically multiple stations around the perimeter of the airfield, that are constantly monitoring for drastic windshifts that could indicate microburst activity. Additionally, weather radar is now essentially standard across commercial passenger aircraft fleets. These have evolved throughout the years, and now are much more capable of detecting windshear and microburst activity. Additionally, training and techniques for aircrew have changed as well. A windshear or microburst warning now a trigger for an automatic go around (climb up and try the approach again), rather than attempting to fly through it.

Finally, one of the crashes/accidents that has had the biggest impact not only on aviation but for everyone basically all of us is Korean Air 007, which was shot down by Soviet fighters after it had strayed into Soviet airspace in 1983. This was one of the biggest factors in the deployment and use of the GPS satellite positoning system. Most airliners, as well as lots of private planes, now have GPS navigation capabilities. The US government used to have the ability to implement "selective availability," or lower precision GPS functions for civilian use. The Clinton administration got rid of this, however, so in some cases accuracy now can get down to the 3m range. Additionally, other additional satellite positioning systems are in place now as well. Russia has GLONASS, the EU is deploying their own system, etc.

I highly recommend looking at the NTSB's (US National Transportation Safety Board) site for some of their findings over the years for more details. Virtually every investigation of airliner crashes has resulted in changes to the planes, the crews, and the equipment.

The worst plane crash in Alaska's history took place on Sept. 4, 1971, when Alaska Airlines Flight 1866 struck a mountain while on approach to a landing in Juneau in poor weather. All 104 passengers and 7 crew aboard the plane were killed, and it remains the deadliest crash in Alaska Airlines history.

Juneau, the capital of Alaska, is a notoriously difficult place to land an airplane. The weather here (I live in Juneau) is notoriously bad, with heavy rain and frequent fog. The airport is bracketed by mountains on two sides, and it frequently experiences severe winds.

The crash and others at Juneau caused Alaska Airlines to invest heavily in landing and navigation aids. In 1989, Alaska Airlines became the first American airline to use heads-up displays in passenger flights. In the early 1990s, Alaska rolled out what's known as Required Navigation Performance at Juneau. That system, now used around the world, allows passenger aircraft to land even in completely white-out conditions. The technology developed in that program is now used in drone and unpiloted aircraft.