The AWS web pages for the EB CLI (e.g. this one) have some good examples on how to start using Elasticbeanstalk from the command line.
However, you may get caught out.
If you get the following:
ERROR: Elastic Beanstalk could not find any platforms. Ensure you have the necessary permissions to access Elastic Beanstalk.
every time you try to run eb init, then the reason is that the user you have previously configured in your default aws profile does not have access to the necessary eb permissions.
The solution is to create a new group, using the AWS IAM console, and then add the relevant policy to it using the "Attach Policy" button. To start off with, AWSElasticBeanstalkFullAccess is a good choice:
Once you have done this, assign your user to this group. eb init will now work as described in the AWS documentation.