Diablo 4 is Blizzard's fastest-selling game yet, so it's no big surprise that the launch of its first seasonal realm is crowded: Diablo 4 Season 1 went live this morning at 10 am PT, and Blizzard says it's seeing "some unusually long queue times" as players pile in.
I didn't encounter a queue myself after logging in from California a little under an hour after the launch, so your experience may vary. If you do get stuck in a long queue, Blizzard says it's working on it.
To participate in Diablo 4's first season, you just need to make a new character. At the end of character creation, you'll be asked if you want the character to live in the Seasonal Realm or in the Eternal Realm, where your standard characters exist. At the end of Season 1, which'll last about three months, your seasonal characters will be transferred to the eternal realm (albeit without any special seasonal attributes).
Season 1's new questline is found after the campaign, which you can skip with your new character if you've already finished the campaign with another character. If not, you'll have to play through the campaign first, but you can do that with a seasonal realm character, leveling them up and earning battle pass tiers along the way.
Blizzard set up this launch with a big patch earlier this week. There haven't been many complaints about the new Unique items the patch added, or most of the bug fixes, but the class balance changes have been very controversial. There's some debate about how hard each class has really been hit, and the new season's powerful gem-like items, called Malignant Hearts, might mitigate some of those changes. Blizzard will livestream a dev chat on Friday to discuss the patch and what it plans to do next.