Welcome to the Coda developer docs - please quickly familiarize yourself with the below links to get a lay of the land.
See the Contributing Guide to start contributing code to Coda. The protocol and CLI are written in OCaml, and the website and Coda Wallet are written in ReasonReact. Any level of experience in any or all of these tools is welcome.
Other documents relevant to contributing code include:
Coda is entirely open source, and the code is distributed under the terms of the Apache License 2.0.