Still a rookie !

Create an IAM users:
a. Master: [ give Admin permission, to have a track of work done ]
b. Readonly:[ give only read permissions, to check all the activities in the account ]
Use "Readonly" users to check the activities to avoid unwanted deletions/action which will cost a lot.
Use "Master" users to keep track of activities done.

1. Check AWS billing report
To see what resources are used. See all the services bill. Get idea in which regions what all services are used/billed.

2. EC2 Instances

Check for the unused EC2 instances in respective region and delete it.
Some instances may not be required all the time but only at particular stage of production. So when these instances are left running at unused hours, costs per hour is just getting added up and in longer run paying for the unused hours is pointless. Hence stop the instances when not in used.
To stop
select instance->action-> stop instance.
Select the right Instance size
Check EC2 instance type. t3 is cheaper than t2. t3 medium per hour costs $0.0448 per Hour and yearly cost will be $392.448 and t2 medium per hour costs is $0.0496 per Hour and yearly cost will be $434.496 which is a lot of difference ie $42.048. Choose the right instance based on processing unit and and amount of memory required. Because your requirements storage might be limited and using oversized resources is lot of waste money, it's just paying the bill for extra resources which was never used in longer run.
To change the instance type of running EC2 instance:
a. Select the instance-> Action -> instance state: stop-> Action->instance type: < select the required t2 types> ->Action-> instance state: start.

3.Volumes

Check for the Available volumes in the region( in short which are not in use)
before deleting, if not sure about it take Snapshot which will be a backup
To snap shot:
select volume-> Action->create snapshot-> give description as required->save
To delete volume:
select volume-> Action-> delete volume.

PS: post will be updated soon with more tips