If it’s a simple split, then use the StringSplit java action in the Community Commons module in the marketplace.
https://marketplace.mendix.com/link/component/170
If your String is actually CSV, with some fields quoted for example, then you may be better looking at the CSV module in the marketplace. This is a lot more complicated to implement, but at lot more powerful.
https://marketplace.mendix.com/link/component/108605
Hope this helps.