I need help with Billing.

What do I have to do to make sure I am not charged?

Don't provision any services and you will never be charged for anything :-)

If this is a brand new account created by yourself (and not under an organisation account for your school) using an email address that has never had an AWS account before, then it's likely to be in-scope of Free Tier.

Free Tier generally lasts for 12 months from the date the account is created. Services in-scope of Free Tier tend to be small, low-powered instances that are designed to get a new user acquainted with AWS and all its services for a low (or no) cost.

The FAQ for Free Tier is here https://aws.amazon.com/free/free-tier-faqs/

Full details of what's in-scope of Free Tier (some items are free for 12 months, some for shorter, some forever, and some never) are here https://aws.amazon.com/free/

Anything that's not listed here, assume it's chargeable. Pricing info for all services is here https://aws.amazon.com/pricing/

Advice on tracking your Free Tier usage is here https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/tracking-free-tier-usage.html

Advice on avoiding charges in Free Tier is here https://repost.aws/knowledge-center/free-tier-charges

If you feel you have been billed for something unfairly, your best course of action is to log a billing support case https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/billing-get-answers.html#billing-support

a code to put towards the AWS account

Interested in what this code is - DO NOT paste it in a reply here. If it is a 15 character string of upper-case letters and digits then it sounds like a credit that you may be able to redeem here https://console.aws.amazon.com/billing/home#/credits