New to Salesforce -> How to use API to Query User to Verify and then Update with new Info?

Question

I'm super new to Salesforce but need to integrate it into a web app. Basically, we need to figure out how to use the API for four things.

1. Query Salesforce and verify a user exists
2. Update that users record if it exists
3. Return info about that user (order number, address, product ordered, etc)
4. Several pages later, update the user again.

I don't know enough about Salesforce yet to figure out this integration, but where can I look for the right information? Or are there any established methods for doing this

Answer 1

https://developer.salesforce.com/page/Salesforce_APIs has all information you need to know about SFDC APIs - SOAP, REST etc.,