Make sure your ECM and coffee grinder are functioning properly. Ideally, have at least 250 grams of the coffee beans you normally use at home ready. A recent roast date and coffee suitable for espresso will make practicing easier, but you are welcome to use your own favourite coffee so you can learn how it reacts to your equipment.
For frothing milk, you will need at least one litre of chilled milk. If there are multiple participants, a little extra milk is advisable. Whole cow's milk is often technically easier to work with, but you can also practice with the milk you normally drink at home.
In addition, make sure you have your milk jug, suitable tamper, cleaning cloths, espresso or cappuccino cups, about two square meters of clear workspace, a power outlet, and running water nearby.