I need to write an Apex Trigger that will check if an account exists when a user tries to create a new account, and either allow the insert if the account is not found or block the insert and display a message telling the user this account cannot be created since it already exists.

I have never written an Apex Code and I will be going to a class in January to learn how to write Apex Code, but in the meantime (since I need this trigger ASAP) can someone provide the code to do this? I imagine this is not very complicated and it is a trigger that many other people may have already written.

Thanks so much in advance to whoever can help me with this Apex Trigger code.