I started playing D&D a little while ago, and the only person that I know that is experienced is my really good friend, so I play with him, but he has a friend who always plays too, and this other guy is like a walking player’s handbook. He has nothing better to do than D&D, so it’s basically all he does and talks about. I use this character creator that is not quite finished yet, so when I make a character and have someone glance over it to see if it was all right. In one instance I was having fun in a 2 player game, him and I as players and my friend as the DM, that I was playing as a Rouge/Warlock in. I was only a level 2 warlock, and some of the Eldritch Invocations don’t have prerequisites to selecting them, and I selected Sculptor of Flesh, which is a level 7 Invocation. We were creating chaos, burning a field, when I decided to use the Invocation to turn into a frog, completely as a joke. It wouldn’t have harmed anyone or anything, then he said I can’t have that. I’m okay with that, he noticed something that I did wrong, but then the DM said that it’s okay for me to have it, he’s the DM, if he says something, it goes, even the other player says that a lot. The other player gets upset at this, and in the same field about 5 minutes later, I say that I drink a potion, well he than said that I take out one of the FIVE sleeping potions that he stuck in my bag earlier. So he was both saying that he did something a while ago, that he never actually did, and then HE controlled MY character into picking out what potion that he wanted me to use. Keep in mind that the DM didn’t even know that he ever had sleeping potions in the first place. I objected, of course, and he says, “Well, apparently if we say that we did something a while ago, it’s perfectly allowed”. I never changed the past, not even once. The only thing that I did that he was upset about, was making a mistake in selecting Sculpture of Flesh and the DM saying that I can keep it.

Another, shorter, instance is when I made a character, and I added something that was homebrew, unbeknownst to me. He complains to the DM about it, and I was almost positive that it wasn’t homebrew, so then I start searching for a page on it. I could’t find it, so I say that I could;it find it, so then he says “See, I told you”. That’s not even what bothers me the most. What bothered me the most is that he constantly uses homebrew in his characters, and not balanced homebrew he found, it’s homebrew he made himself, that tailors to his needs. He does stuff like this constantly. Every. Single. Game. Even if there isn’t anything wrong, he finds something. Our group always ends up ending a campaign after the 3rd or 4th section because of this, and it’s ruining the experience that I was brought into D&D by. I really want to keep playing D&D with my friend, but I can’t stand him anymore. Any ideas on what I can do?

Talk to the DM.

Tell him to F off and get a life.

Few things come to mind.

I'm sure his side of the story would be quite a bit different so I am going to assume reality is somewhere in between as you seem to be nitpicking his character too (maybe not to his face, but to us).

Stop worrying about what his character has.  Let the DM handle it.  As a beginner, simply enjoy learning to play YOUR character.   Take the feedback from the other player graciously and learn from it, (If his tone doesn't improve after that, then he is just too immature to want to play with and you'll either have to suck it up or find a new group.)

Ask yourself if you can still play and have fun with this otherwise nice person. If not, you should leave.

I agree, there are two sides to the story. I have two suggestions.

First, for both of you, get any home brewed options approved by the DM prior to using them. That means outside of playing time so your DM has time to review them at his leisure instead of being put on the spot. The DM should insist on that anyway because home brewed options can seriously unbalance a game. A DM in a game that I’m playing in gave my character a home brew magic item that’s going to unbalance the game slightly for the next several months until he gives similar items to the rest of the PCs for example. But he told me that he has similar items planned for everyone else which will make everything balanced again.

Second, learn the rules about your character as best you can outside of the game from D&D Beyond or the official physical books, especially if you’re using a character generator that isn’t complete and doesn’t fully follow the rules. Using abilities in ways that are different that how the rules are written will unbalance your character and make the other people who you’re playing with have less fun because they won’t be able to contribute the way you’re contributing. Similar other player using unbalanced home brewed options that make his character unfairly powerful is making you have less fun.

Talk to the DM and the other player and try to work it out calmly outside of the session. Good luck!

Like most problems with humans, the best way to work through them is to talk it out. I know it is difficult to work through conflict with friends but sometimes its necessary. But at the end of the day if you talk though it and it doesn't work out, just don't play with that group. Not every person is going to fit every group, and even if you aren't the odd one out, if someone else is causing a problem sometimes its just better to walk away before it becomes something else.

As far as his saying he did things awhile ago that apply to your character, two can play that game. If says he put sleeping potions in your backpack "a while ago", but didn't tell anyone, including the DM, about it until that moment, immediately say that you took the potions out "a while ago" or otherwise noticed them.  Fair is fair, and if one player is allowed to freely retcon the game every player should be allowed to.

Also talk to your DM. It sounds like this other player is making the game seriously not fun for you, that is not OK.