Im new to salesforce, I have an Object Training__c and field End_Date, and when the End_Date came I need to create a task, but I dont know how to track this End_Date, because it is not a trigger...
Thanks
Look into time-based workflows (bit old school, we're encouraged to use flows now so check scheduled flows out.
It could be a whole scheduled flow (kind of like a nightly batch job) or a scheduled path in your "normal" flow (if you already have one on this object). There are some trailhead modules to get you started:
Roughly speaking you'd set the action to fire "0 days after end date" and it becomes Salesforce's problem to modify the job if the end date changes. It's elegant, code free, fairly easy.
There are some problems with it such as scale, will there be tens of thousands of records? Another thing is this will work only for records created / modified since you activated this flow. What about all old data? What if I need to modify the flow's definition, will it de-queue all actions? (that one was a legit concern with time-based workflows. to edit the workflow you had to deactivate it - but doing so nuked all submitted actions).
So... you may decide to write some code for this after all. Have an apex batch job running nightly, selecting records with End Date <= TODAY that don't have (open)? tasks yet - and adding these. (maybe if a task is completed you'd want to give another one). Different solution, requiring you to write an unit test for it too (which isn't neccessarily a bad thing), bit more resilient than flows.
This looks like fairly similar problem solved with a batch: https://salesforce.stackexchange.com/q/118214/799