Hello and welcome to the forum,
One way is to wade thru the IBM documentation. Tedious at best. . .
Another way is to buy a book (there is a good one at Amazon.com - Rexx Programmer's Reference (Programmer to Programmer)).
If you are working on some code and get stuck, post what you are trying to do and your question/problem. Someone may be able to offer a suggestion.
We are not able to "train" - it is simply too big a task.
Hope this helps,