How do I get told which items to reorder in Aicountly Inventory?

Set the stock levels on each item, then read the Replenishment report. On the item's form, the Stock levels section has the fields Minimum, Maximum, Reorder point and Reorder quantity. The reorder point is the one the replenishment reports compare your balance against.

Open Replenishment in the registers (analysis group). It lists items at or below their reorder point after taking reservations, pending outs and expected receipts into account. The columns show On hand, Available, Expected in, Pending out and Projected, a Reorder? flag, and a suggested quantity and value. By default only the items to reorder are shown; you can also filter by warehouse, item group or stock category, and the Inventory dashboard shows a count of items at or below their reorder point.

Items with no reorder point set will not be flagged, so fill it in for the items you care about.