Sadly, the Newbie Guide (fan product) has never been completed, so the best online resource at the moment is the Vaults of Pandius that Inferno! linked to. It can be a little disjointed, though.
If you have them, S'Mon's suggestions of X1 (with its brief overview of the Known World that, for many years, was the entirety of the setting) and Gaz1: Grandy Duchy of Karameikos would be the best choices.
The Poor Wizards' Almanacs give a good overview of the entirety of the world as it has been developed, but can be pretty dense reading and probably not for the uninitiated (especially given their combining of the Hollow and Known Worlds). A better newbie alternative would be Joshuan's Almanac, which excises the complexity of the Hollow World and provides more colorful and descriptive introductions to the various nations in lieu of the cold facts of the PWAs.
The Kingdom of Karameikos boxed set is actually a pretty good newbie product as well, because it provides a general overview of the world and setting, in addition to its extensive handling of Karameikos (the focus, as one might guess). (It's not quite as useful for experienced players, due to basically rehashing all the material from the Gaz, and for the half-hearted conversion to 2E, but all of that really had to do with the designer, Jeff Grubb, being handcuffed by TSR execs- he wanted to do a world book and advance the plots more in conjunction with what went on in the Wrath of the Immortals, but wasn't allowed to.)
So, to recap:
- X1: Isle of Dread - Brief overview of Known World setting.
- Gaz1: Grand Duchy of Karameikos - In depth on one nation (main newbie setting), brief info on surrounding nations.
- PWAs - Atlas type overview of Known and Hollow Worlds, possibly too in depth for newbies.
- Joshuan's Almanac - Colorful in-character descriptions of Known World, along with NPC info, and select other flavor from the PWAs, and some description of the year's timeline (without the in depth and sometimes overly descriptive text of the PWA years' events)
- Karameikos: Kingdom of Adventure - In depth on one nation, and a decent overview of the surrounding nations and world of Mystara.
Also, you could do a lot worse than ask around on the
Mystara message board at Wizards.