I'm starting a new campaign soon, and I've managed to get 4 friends to agree to join. One of them is an experienced player and the others have never touched a d20 in their life. The experienced player(Player A) wants to choose races such as Klashtar, Triton and wants to pick classes like artificer and blood hunter. I feel like letting my whole party choose from all these races and classes may overwhelm them, since they're just starting out, but I also feel like letting Player A have his choice and restricting the others is going to be unfair. What do I do?

Just have a chat with the experienced fella. More than likely they'll see your point and play the game your way until the others get more accustomed.

Letting player A have his choice and restricting the others is unfair.

Either you need to let them all choose from all possible races, or restrict races for everyone. Personally, for a starter group, I would recommend limiting them to "races, classes, and subclasses in the Player's Handbook only." The veteran player ought to understand this, and if he won't cooperate, that tells you that you have a problem right from the off.

What we did was let the experienced players do more or less whatever they wanted while guiding the new players through character creation.  Instead of presenting all the options, ask them questions to suggest classes and races to them.  Even narrowing it down to martial/caster/mixed is enough to only have to present a few options.  The main point is making sure the new players only have to learn about their race and class.

My spin would be to allow the experienced player their choices (although at my table Artificer and Blood Hunter are both a "nope" class)   I would also allow the others to pick the more complex combos, with an understanding that it MIGHT hinder their fun some, trying to learn on a complex character.  I would encourage them to pick more mainstream setups, letting them know that as DM you will MAKE all the classes and races viable through the campaign.

Perhaps even slate your first adventure to be shorter, ranging maybe 4-5 levels before ending.  This will allow all to get their feet wet as it were and better understand the game mechanics.  Some might go for a more complex setup on a second run, while others might be enthralled with the game and not want to up the complexity of their character.  Freedom with guidance, I guess, is what I would suggest.

I recommend going for a few one shots or a very short campaign that last no longer than two to four sessions. A regular campaign is way too long for new players, and you want to give players who do not like D&D a chance to leave and quit without affecting the party too much. Running one shots and short campaigns also let new players learn the game without having them to commit to a character that they might not enjoy playing and can switch it up during the next session.

Personally though, I would just have everyone choose pregenerated characters and delay character creation until after the first campaign. In my experience, creating characters takes a long time for new players, so it is better just get the action going and have them pick pregens for their first one shot/campaign. If you do want them to create their first character for their first campaign, I recommend having everyone create an account on Beyond but disable content sharing and UA, so everyone can choose from a limited number of races and they have no option for subclasses.

Quote from Lunali >>

What we did was let the experienced players do more or less whatever they wanted while guiding the new players through character creation.  Instead of presenting all the options, ask them questions to suggest classes and races to them.  Even narrowing it down to martial/caster/mixed is enough to only have to present a few options.  The main point is making sure the new players only have to learn about their race and class.

That's exactly what I did with the campaign I started last fall, and had the same kind of experience mix as OP -- we have two experienced players, one on her second real campaign, one who'd played nothing but a couple one-shots and one who's never played at all.

For the latter two, I asked them what sort of character they wanted to play in general terms, and then offered suggestions on how to get there (i.e. "Basing a character off Stevie Nicks is cool! That could be a bard, but you could also make a sorcerer or warlock with the Entertainer background if the witchy element is more important to you than the singer element")