Bruce is correct that new IFTTT integrations have been turned off. There is no eta for when/if it will be turned back on. Given the huge latency involved when using IFTTT, it’s not really that big of a loss. In addition to the hosting your own instance of the openHAB Cloud server as suggested, I think the Webhooks channel in IFTTT can work with myopenhab.org, but can’t say so for sure.
In answer to your original question, the best place to start is the docs, in this case https://www.openhab.org/docs/configuration/rules-dsl.html. After reading that if you have specific questions we will be happy to answer. How to get started (there is no step-by-step tutorial) also has a number of getting started resources.