how to back up my ec2 instance for a biginner

Does your EC2 instance have only instance store or does it have an EBS volume - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Storage.html

If you already have your data in EBS, then your data won't get lost if you reboot your EC2 machine. However, it is still recommended to backup the data in your EBS volumes - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSSnapshots.html

If you have only instance store, then the data is ephemeral. Then you should create an EBS volume and attach it to your EC2 machine and copy the data from the instance store to the EBS volume, as mentioned here - https://aws.amazon.com/premiumsupport/knowledge-center/back-up-instance-store-ebs/

There is also a service called AWS Backup that allows you to backup your EBS volumes among other things. This tutorial will help you come up to speed on how to do that - https://aws.amazon.com/getting-started/hands-on/amazon-ebs-backup-and-restore-using-aws-backup/

Just take the snapshot of the block storage and terminate the instance. You can able to restore it in the future with the same configurations.