You don't need to know that much to give it a try,
PFS ID numbers are for you to track your characters. If you don't have one beforehand, you can get one onsite.
Pregens are available at levels 1, 4, and 7. Most PFS scenarios are for a group of 5 levels (1-5, 3-7, 5-9, 7-11) and then divided into low or high based on what characters show up. (So a 3-7 might be in the 3-4 or 6-7 subtier.) Practical upshot: you can show up for almost any scenario and play a pregen. My recommendation is to stick to the low end if only because they're mechanically simpler characters, but it's your choice.
If you have the time, it's better to make your own character(s). You can read about the factions and other special rules for PFS characters in the free-to-download PFS Roleplaying Guild Guide. Factions only matter for characters you create; all the pregens are considered to be in the Grand Lodge faction and cannot earn special faction rewards anyway. (Unless things changed in Season 7.)
The big interactive special (where the whole room succeeds or fails together) on Friday night is a favorite, but don't play that with a pregen. One limitation of PFS is that you can only get credit for playing most scenarios once, so it's better to play these sorts of things with a character you know and love. :)
If you're looking for a starting point I'd recommend playing The Confirmation, which is designed as an intro to the Pathfinder Society for new players and characters that you can play multiple times. The large Thursday and Saturday night events are ones where everyone plays the same adventure with the same set of pregens, so those are decent for jumping in if you're fine with the mechanics. (Story-wise you might want to wait to play in those until you know more about the PFS world, but as mentioned it makes it a lot easier to get seated at a table.)
Hope this helps.