So I paid for a minecraft account and everything but when I try to log in it says that I don't have a username. Is it an email or an email that you've made just for this account?
If you have just bought Minecraft, you account will be a Mojang account. This means that instead of using your username (ex: player1234).
In the username field, you will use your email: (eg: firstname.lastname@example.org).
When logging into Minecraft with a Mojang account (or a migrated Minecraft account), please be sure to use your email address, not your username, and Mojang account password
Minecraft Login servers could be down (they aren't at the moment of this post, you can always check at help.mojang.com)
You could be using the wrong username. If you created your minecraft account after a certain time (after they changed logging in with your username to logging in with your email address) then you will need to login with you email address and whatever password you created when you created your account. "To create a username, register a Mojang account at account.mojang.com. After you are logged in, you can purchase a Minecraft account. You will be sent an email with a redemption link, which you can click to complete this process. If you do not click the redemption link, a green box will appear on your Mojang profile, prompting you to choose a new username, or import a previously-registered one." (https://help.mojang.com/customer/portal/articles/928638) That could also be the problem, that you haven't redeemed your purchase and/or selected a username. You should check your email and see if you need to still create your username.
You do NOT need to migrate your minecraft account to a mojang account if you do not wish to, although a Mojang account will have more features, and you are required to use a mojang account if your account was created after they decided that.