If you want to use the AdWords API to programmatically interface with AdWords, you should:
Consider which programming languages and development environments best suit your needs.
Download and experiment with the appropriate SOAP toolkit.
As you create your programs, understand the hierarchy of AdWords API objects: keywords require AdGroups, AdGroups require campaigns, and so on.
Create one authentication header to use with all the AdWords API web services you call.
Encapsulate as much code as you can in functions placed in library files that can be shared between modules so that you can reuse code.
Make sure to carefully validate user input before submitting values to the AdWords API web services.