How Do I Begin?

It’s up to you! You could begin by pointing-and-clicking your way through Configuring Salesforce1 and working all the way through Using Actions in Salesforce1. Then, put your developer hat on and create custom actions, or add canvas apps or Flexible Pages to further enhance your users’ Salesforce1 experience.

If you’re a seasoned developer, you could jump into the deep end first and start with Salesforce1 development in the Salesforce1 App Developer Guide.