The easiest way to create an EC2 instance perfectly suited to your purpose is to start from an existing AMI that is already close to what you need, modify it to do what you need, then save it as your very own image by bundling the running instance into a new AMI. When you bundle a running instance, the entire root filesystem is stored in a new AMI, along with the software and configuration changes you have applied.
In Finding Amazon Machine Images” in Chapter 5, we demonstrated how to find, launch, and log in to one of the public AMIs made available by Amazon. Amazon’s public AMIs include only a small set of tools and applications, which makes them a good starting point if you want to customize your own image from a minimal base.
Alternatively, you can choose from many public AMIs provided by other EC2 users that already include the software and configuration settings to fulfill common needs. There are many preprepared images available that include ready-to-run environments for things such as cms or web application server environments for Ruby on Rails and Java. Unless you have unusual requirements, there is a good chance you will be able to find a public AMI that already includes everything, or almost everything, that you need. The vast majority of the public images are free.
Find more information about the public AMIs provided by Amazon and others here: http://developer.amazonwebservices.com/connect/kbcategory.jspa?categoryID=101.
When you use ...