In order to learn AD&D, you have to read the rulebooks. There isn't a shortcut to learning the game.
If you know someone who has read the rulebooks, you might be able to start playing right away by learning from them, and then you can read the rulebooks steadily in your own time. But in practice, very few people play the game by the rules. We've found that many people who say they run AD&D have not read the rulebooks, or ignore many of the rules, or play some game that is not AD&D, such as OSRIC, OSE Advanced, Hyperborea, or a mashup of AD&D and “AD&D 2nd Edition”. Make sure you're really learning AD&D if that's what you want to do!
AD&D has so many unique rules that knowledge of other versions of old-school D&D does not give an understanding of AD&D. People who have been playing the game for 30 years discover rules they didn't know exist, simply because they never read through the rulebooks.
When reading the rulebooks, you can safely skip over the spell1) and magic items descriptions, and you certainly don't need to read every monster entry. These things you can look up during prep or play. But everything else should be read – even the stuff you think you know from other old-school adventure games.
Demon Idol is intended to be used in conjunction with the rulebooks. If you've already read the rulebooks, you can go ahead and start reading through the links under the “Rules” action of the sidebar over at the left of this page. Those pages contain various annotations and explanations of the rules, especially in cases where the rulebooks aren't clear or fail to provide gameable material. Or if you're curious about a particular rule or ruling, you can use the search feature at the top of the page.
If you've never read the rulebooks, it's time to get started. DMs should read the PHB, DMG, and rules content of the MM (see Rulebooks Guide). Players can survive with a look at the PHB if they have a friendly DM willing to teach as they play. As you're reading, you can check Demon Idol for rules clarifications, or follow along in Demon Idol with the corresponding section of the rulebook you're presently reading.
Over time, we plan to add more helpful tutorial and instruction material to Demon Idol, along with sheets and generators.