Knowing your league starts by understanding the league settings. Know all of the rules before you start building your auction plan because those rules will help to shape your strategy. Get a copy of your rule book and go over every part of it, from the league size (number of teams), scoring setup and lineup requirements to how the entire season and league operates. Start of course with the auction settings, seeing what the budget for each team will be and how nominations will work. Next, move on to the season long settings and take note of how teams will square off with one another. Do teams play each other head-to-head on a weekly basis, or is it a total points league or Survivor format? Do you have to set a lineup every week or do your best scores on your entire roster count? Is there a waiver wire, or is it just a pool of free agents? Do you get to grab anyone you want or do you have to pay for it (literally or with another pool of fake money like at the auction)? What about trades - are they permitted? Can you trade during the auction? Can you trade auction money?
All these things can have an impact on how you value your players before auction day and they all factor in to the strategies you will use on Auction Day. Study up on the league and get prepared.