I'd start here:
https://create.meraki.io/learn/
Start with the one on the top right (Intro to the Dashboard API) to get your first steps going. There's a lab exercise section in it that provides an API key to a sandbox:
Lab Exercises
This exercise will get you started with the Meraki Dashboard API using a free tool, Postman, to make REST API calls to the Meraki dashboard. Answer all questions using Postman and API calls, no Dashboard use please! You can access the latest API reference docs here: https://api.meraki.com/api_docs
Use this API key: 093b24e85df15a3e66f1fc359f4c48493eaa1b73
- Determine the Organization ID for the “Meraki Launchpad”
- Find out through the API when the license of this organization expires.
- Is SNMP enabled on this Organization?
- What is the value for “claimedAt” for the device with serial number “Q2EV-D92Y-TX32”
- What’s the name of the network that contains the device with serial number “Q2HN-7N9M-89PH”
- What tags are applied to device “Q2UD-7N75-UGU5”?
- How many VLANs are configured on device “Q2QN-5DPW-DXQT”?
Then take a look at the devnet labs linked on that same meraki.io/learn page. Those will also show you how to use the Scanning API and the Captive portal API among other things.
After that you're ready for pretty much anything. There are lots of other applications and tutorials on create.meraki.io and meraki.io in general.
Also, register for a meraki Webinar so you get your hands on some free hardware so you can test with your own environment:
https://meraki.cisco.com/webinars