My friend and I are both new to DnD and we are using DnD-Beyond to make our character sheets. It is a Homebrew as we are making a Pirate campaign. He wanted to make a Alchemist of the Venomist School (Idk what this exactly is), but he has no clue on how to build it. Any information on how exactly to make this build would be perfect for us https://xender.vip/ https://nox.tips/ , as without his sheet, we cannot start.

I have no earthly idea what the "Venomist School" is supposed to be, but Alchemist is a subclass of Artificer, and you can either get the class in the Tasha's Cauldron of Everything book or possibly just buy the class itself. There's a handful of poison/acid based spells, I think Wizards get access to most of them, but elemental specialization gets really limited and spotty support in 5e, and poison and acid in particular don't have much going for them.

Now, as a disclaimer for first time players, it's really best to not try and base your build on a specific character from a different piece of media, especially one with its own magic system. D&D is a hard RPG system as opposed to something softer and more cinematic, so it can be really hard to shoehorn a completely foreign concept into it. Not trying to just come in and crush dreams or anything, but this is very much a "manage your expectations" area, especially for new players. Some friends and I tried to homebrew rules to adapt an anime we all enjoyed into the D&D system, and it was so-so but had some very wonky areas when it came to translating the show into the game.

This is kind of getting into some technicalities of the game, but here's how I would go about creating this and why...

First of all, in order to utilize poison you need to have proficiency with a Poisoner's Kit. There actually aren't a lot of ways to get this, since it's not technically an artisan tool... you could take the House Agent background, or just take any background with a tool proficiency and use customization rules to replace the proficiency granted with Poisoner's Kit. One of the easiest ways, though, is to take the Assassin Rogue subclass. However, the problem with that is that, in order to attack with poisoned weapons, you need to apply poison to a weapon, which is usually a full action. So for the most part you can apply the poison before combat starts, and you'll be able to poison just one person. Then if you want to keep using poison through the battle, you'll basically just be attacking every other turn.

So, all that explained, I think the class that would work best for a dedicated poisoner would be the Thief subclass of Rogue. The Thief Subclass has the "Fast Hands" feature, which lets you perform Object interactions that normally require the use of a full Action to be just a Bonus Action instead. I'd take the Criminal Background, and just swap out the Thieve's Tool Proficiency (which you get for being a rogue anyway) with Poisoner's Kit proficiency. From there you can mostly play as whatever Race you feel like, but one that stands out to me is the Yuan-Ti... you might have to buy the race separately, but they have a feature where you can cast Animal Friendship on snakes an unlimited number of times per day... this can mostly be used to find venomous snakes, charm them, harvest their venom, then use it as Serpent Venom for your weapons.

If you're new, don't do home brew. I know it's tempting. But you'll probably have a much better and simpler time understanding the game without (potentially over or under powered) homebrew getting in the way of your fun. 

Also, I totally forgot about the Poisoner feat... it does the same thing that I was recommending Thief for, in that it lets you apply poison to your weapon as a bonus action. It also gives you Poisoner Kit proficiency and lets you ignore resistance to poison when attacking enemies. So if your friend plays as a Variant Human and takes the Poisoner feat, they can basically play as whatever class they want. It would pair best with a martial class, since it benefits someone more when they're skilled at making weapon attacks.