No one should ever intentionally try to get to Ravenloft. That is an extremely dangerous proposition, and if you do it, you should not anticipate ever leaving.
If you really, really wanted to, the Demiplane of Dread is, for someone going in, “just” a demiplane. You can get there the same way you get to any other demiplane, usually the plane shift spell. For that matter, if you felt like traveling through the Deep Ethereal (which under normal circumstances I’d call insane, but we’re talking about someone who is intentionally trying to enter Ravenloft, so that’s a moot point), you can just walk in.
And on very rare occasions, Ravenloft experiences “conjunctions” with the Material Plane. On such an occasion, you could walk into Ravenloft from Toril directly, no spells, no ethereal travel, just walking. You might very well not know you had done so (actually, probably wouldn’t, since if you’d known you should have avoided it). Most of this is extremely rare, and completely random, but it’s possible to imagine certain very specific locations with more regular patterns or even more frequent conjunctions—something someone could actually use intentionally. Such opportunities would be rare and very, very difficult to track down, however—and ultimately entirely up to the DM.
As for who can enter, Ravenloft is mostly not picky (about residents; it’s very picky about darklords). The only people Ravenloft actually has a problem with are outsiders—celestials, fiends, and so on. They seem to bother the plane or its Dark Powers or both (or at least so we assume). It will try to keep those out, and if they get in, it will often kick them out again at the earliest opportunity.
That sometimes applies to very-powerfully-aligned mortals—merely being good doesn’t do it, but high-level paladins (which in previous editions were the most strongly Good class in the game) have been known to get the boot. We might assume certain high-level sorcerers (divine soul) and warlocks (celestial or fiend patron) could get similar treatment (though most Ravenloft material predates either class’s modern incarnation, so that’s speculation on my part).
There have been exceptions to this, though. Ravenloft has kept a few outsiders—more fiends than celestials, but there are some of both. And trying to get some personal gain from a trip to Ravenloft sounds to me like a good way to get the plane to decide to keep you. The reason we know that you can just walk into the Demiplane of Dread from the Ethereal is because a certain celestial did it, chasing a fiend—and she’s still stuck there, set up as a Darklord with a traveling domain, the Carnival (her curse is that magical posters appear well ahead of the Carnival’s arrival anywhere, so her quarry can always flee before she gets there).
But other than Ravenloft kicking you out, as far as leaving is concerned, that simply isn’t going to happen. Only two characters have ever managed to free themselves from Ravenloft, and the only one to actually manage to stay free, Vecna, broke reality so hard in the process that it changed the AD&D 2e rules into D&D 3e rules. You can leave Ravenloft through a conjunction, but since those are under the control of the Dark Powers (probably), that is basically just a more polite way for the plane to kick you out.
The other exception I mentioned was when Azalin managed to manipulate the plane into creating a “grand conjunction,” which actually freed a number of darklords, including Strahd, who you might recognize as the current darklord of Barovia. Azalin, likewise, is still the darklord of Darkon. That’s because Ravenloft’s hold on its darklords are more than just its magic. The very flaws that got them imprisoned in the Demiplane of Dread in the first place caused them to ruin their own opportunity, and get themselves recaptured. Really, there was no way it was going to turn out any other way: you simply do not get out of Ravenloft. To put things in perspective, getting someone out of Ravenloft is well beyond the abilities of even divine intervention. Vecna is literally the god of cheaters, and he certainly couldn’t manage a repeat performance.
On the other hand, the mists of Ravenloft are known to scoop up adventurers from time to time, and basically bribe them with freedom in order to get them to do something the Dark Powers (presumably) want done. This is the source of a great many Ravenloft adventures, and pretty much the typical plot hook recommended to DMs who’d like to take the PCs there. The alignment of the adventurers makes no difference to the mists, and oftentimes good adventurers are preferred because the occasional “good deed” is exactly the kind of blind spot that the Dark Powers have (probably) and need outside assistance for.
