It’s really hard to help with general “how do I do this” type questions- you’ll have better luck if you post a specific “I tried X, expected Y, but got Z instead” type question.
The best advice I can give you is to break your problem down into smaller steps and take those steps on one at a time. For example, can you create a small example sketch that uses a hard-coded ArrayList and loops through it?
Shameless self-promotion: here is a guide on using ArrayLists in Processing. You should also check out the Processing reference and the examples that @deerfeeder mentioned above.
If you’re still having trouble, you can post a more specific question along with a smaller examples that shows where you’re stuck. Good luck!