(This time, on the right account)
I have a few questions about creating a campaign setting based on western movies. I have a few questions:

How do I do guns(I’ll probably home brew them)?

Should I do fantasy races?

How on earth do I do classes?

There are guns incorporated in the Dungeon Master's Guide, as well as a Gunslinger Fighter subclass that's available for free on D&DBeyond. There are two easy ways to do races. First, everyone is a human, and other races are just reflavored to be skinny or young or whatever humans. Alternatively, and perhaps more interestingly, you can incorporate those races into your setting without destroying the genre. Maybe a tribe of gunslinging orcs dominates the pass between two towns, robbing those who pass through the area. Perhaps a clan of dwarves is searching for riches beneath the sand, but their mining inadvertently sets off a small earthquake. The possibilities are endless.

You can go in a similar direction with classes. You can either ban full casters (but not artificers or rangers, they definitely fit in), and have everyone play as half-casters or non-mages, or you can have magic be a part of the setting. Maybe magic is rare and mysterious, so it's not trusted by most people. Explorers and adventures come to the desert to search for ancient artifacts buried beneath the sand. Just remember that you're in charge of the world. Just because westerns traditionally don't really include magic doesn't you can't make it a minor or even major part of the setting.

There are optional firearm rules, or you can re-skin bows become rifles, for example. You could also re-skin magic as technology. Re-skinning is your friend.

I've actually created a more or less fully fleshed out Wild West setting, and am actually currently DMing a game in the setting on these forums. For firearms, I am using the firearm rules from Mage Hand Press' gunslinger class, which I have found to be the most balanced firearm rules out there, far better than the hot garbage in the DMG. I wouldn't recommend attempting to avoid magic or fantasy races entirely. The setting can certainly be low magic, or perhaps firearms are magically powered, but a D&D game without magic is like a chicken without a head. It might not die immediately, but it will soon enough, and any existence it has will be empty and miserable. As far as classes go, I wouldn't change a thing. Really, all you need is some good firearm rules, and you're set.

Is there any way I can see your setting?

Well, I can offer you the very much unformatted worldanvil link, if you like: https://www.worldanvil.com/w/the-wild-waste-woobydoobydoo

Another thing to consider is building the western frontier into your normal setting - a land where tribes of "savages" do things like understanding the land, communing with nature, and being generally peaceful, and therefore need civilising!

Perhaps have a resource out there which powers firearms, thus causing the equivalent of a "goldrush" and also justifying firearms.

Cherry pick the bits you feel "make" the westerns - wide open spaces, guns and horses, natives being kicked off their own land by other people. Maybe even make your own race for the natives (or pick one not normally in your world) so they are distinctly different.