How can you insure that a character you create works in one specific realm?  For example I need a character for a Forgotten Realms game on Saturday.  How can I make sure I'm only using those rules?

Thanks in advance

Speak to the he DM. 

I believe the PHB us applicable to all official settings, however, the best thing is to ask for a lot at of accepted sources from the DM. They get to dictate what is and is not accepted. If the DM says Warforged are allowed, then they're allowed, even though they're not part of FR. Conversely, if they say there are no Elves, then there are no Elves.

Ask the DM for what's acceptable. We can just give you guesses.

if you are asking how to using the DNDB character sheet.  Then best i can guess is to uncheck all source options when you start your character creation.  That eliminates most of the non-core stuff, but the lore agnostic entries will still be available.   This is also assuming you either own or are in a campaign where someone owns a number of sources they are sharing.

Don't use anything for Eberron, Wildmount, or Mystic Odysseys of Theros.

Asking your DM on what sourcebooks are allowed is always the best option though

There is no real difference. It is just a setting.

Think of Forgotten realms as a theme park.

If you have only ever gone to Six Flags you can go to Busch Gardens.  The decorations are different but you can still ride the roller coaster without doing any thing different.

It really doesn't matter. You can justify anything in any number of ways and make it coherent.

Realmsmith (streamer) had a warforged in Curse of Strahd. So a robot man in vampire town. Doesn't seem like a good fit, right? At least that's what I initially thought. But the warforged was portrayed as a little boy who's spirits was put into a construct by his father to save him. It worked wonderfully and was arguably the best character of the campaign.

You can make anything work. That's the beauty of the game. You just need some imagination and a open minded DM.

Hope this helps.

Forgotten Realms is intended to be a standard, general-use D&D world. What that means is that almost any concept that springs to mind from the rules will probably make some amount of sense in it, because it has all the common D&D things, along with most of even the weird/specific ones.

If you're going in with no prior knowledge and want to play it safe, I'd say, like... if you make a character using options and information from the basic rules or PHB, there's very little chance you'll make anything out of place. I'd be a little wary of inventing specific important locations or large organizations for backstory, but even those sorts of things can be incorporated easily or switched to a similar canon idea once the DM gets a look at it.

Like, for FR especially, don't feel like your hands are tied. But if you're looking for something that clearly fits in, the core rules are sort of tuned to Forgotten Realms, so it's hard to go wrong with any of the base options, and you can assume that the general descriptions of things like species and classes are based on how they exist in FR.

but honestly, if a plasmoid artificer from barovia speaks to you and the dm doesn't want you to play it, they're just a coward-