I think the best thing to do is ignore everything before. When you find your feet, then go back. Its not as complicated as it sounds, honest.
Its basically:
1958 (pre-boot legion) The longest running legion that ran into the 80's. This is the Legion Johns uses. After the crisis Supergirl and Superboy were retconned but it took a while for Levitz to accept that. This Legion runs until the end of the Sensor girl arc in the 80s (but that is up for debate)
Levitz tried to continue by replacing Supergirl with Laurel Gand and Superboy from a pocket dimension (the same one matrix supergirl came from). This failed with readers. Superboy was killed and Giffen decided to jump ahead 5 years in the stories for a fresh start. This is called the 5 year gap. It was depressing but told some great socially conscious stories. It ended with the world exploding and the Legion replaced by younger clones All this paragraph is deemed out of continuity from the 1958 legion. It was originally in continuity but has been thrown in the bin.
Post Zero Hour Legion. A New Legion coming out of the Zero hour event. Nicknamed the Archie Legion because they were so young and their stories so fun filled. It was like the WB cartoon actually. They were awesome because they got back to the silver age ideal of the legion where the 5 year gap went completely the wrong way. They lasted for 10 years and had multiple books.
2005 Legion The Threeboot, all about Teen Rebellion and young people being politicised. It split fans down the middle. They stopped telling stories about this team in January.
My advice, start small. Everyone needs to start somewhere with Legion or no one would still be fans. Watch the cartoon, get familiar with the main players. Read Legion 'boots independantly of one another. If you start with the threeboot then imagine no other Legion exists. The threeboot is the most new reader friendly because it starts each comic with a page showcasing the main characters in that issue (names, planet, powers) and its all in trades. Or you could start with Adventure Comics which will have an origin and omen I guess for each legionnaire starting with Starman then going to Lightning Lad and Sun Boy and Polar Boy to build the team for new readers.
Don't try to know everything, no one expects you to because there is so much and if Johns is worth his pay cheque he won't refer to the past without an explanation.