Free Salesforce account
Having a paid account is not necessary to learn Salesforce API.
Salesforce provides usefull free tools to learn everything on the CRM and his API: https://trailhead.salesforce.com/
Create a developer account is totally free: https://developer.salesforce.com/signup
Configure API access
Click on "setup" -> (Build) Create -> Apps
Then you can create a connected app to have something like this:
Last step is to generate security token and receive it by mail.
Click on "My settings" -> "Reset My Security Token"
You are ready to use API
You now have:
- Clientid
- Clientsecret
- Securitytoken
- Username (your email)
- Password (used to login) -Instacename: (Salesforce domain prefix)