If you are under the impression you have already perfected yourself, you will never rise to the heights you are no doubt capable of.
Here are a few things to think about:
Why (in universe) does the character die? Is it an accident? Is it a random stranger who mugs them, hits them by a car, etc.? Is it murder? Is it part of a war? Whether or not the death is intentional in-universe is important when it comes to writing a death scene.
How does the character die? Obviously, some of these questions are answered by answering the previous question, but even within the realm of murder, war, mugging, etc., the how matters. Think about what weapons the character will have. Think about what weapons whoever kills them (if someone kills them) has. If they’re poisoned, figure out how they ingest the poison. Figure out all of the little details, especially if you show the death scene in the story (as opposed to just talking about it).
How long does it take them to die? If the main character (or anyone else) is there, it matters a lot how long it takes for the best friend to die. If they die instantly (and without warning), nobody has a chance to say goodbye. If they take a long time to die, the main character (or anyone else) would be able to talk to them.
Why (in meta) does the character die? Essentially, why do you want to kill them? Is it an important plot point, or are you just sick of writing them. If the only reason that you’re killing them is because you’re sick of writing them, you should seriously reconsider having them as a character at all. Similarly, if their only reason for dying is because you need some reason to have the main character’s personality change, make sure that this isn’t just because you got sick of having the main character act the way that they do. Make the death important, because otherwise people will think it’s stupid.
What was your character’s feelings towards them right before the death? I know that you said that the character is the best friend, but people aren’t 100% totally blissfully happy with their best friend 24/7. Did they have a fight? Was there something left unsaid? Were they blissfully happy as best friends?
Who else is the character leaving behind? Just because they are the main character’s best friend, it doesn’t mean that they don’t have other people in their life. They probably have other friends, family members, possibly lovers/significant others. Their death should affect people other than just the main character.
Make the character a real character before you kill them. Especially when you want the death to be a sympathetic one (which I assume you do), you need to make sure that the readers care about them before they day. Don’t just make them important in the context of the main character. Make them important in their own right. Then their death matters.
Kill them at the right time. Or, rather, kill them at the wrong time. Kill them when it hurts the most for them to die. Kill them just when the main character needs them most, just when they’re about to do something important, just when they’re about to change their life. Doing it when people have the most to lose. That’s when it matters the most. Obviously, this isn’t the only choice, but this will have some of the greatest emotional impact.
If you have a question with more specifics, I can give you an answer with more specifics. Otherwise, I hope this helps.
This is not a problem with young writers; it’s a problem with inexperienced writers, who are also often young. It gets attached to teen writers especially, whether or not that’s fair.
Here are some links that can help you with structure:
I recommend checking out books in your genre of choice (and other genres as well) and take notes as you read. Look for:
One big thing that all novels hinge on our strong characters. Here are some links that can help you with that.