Everything you need to know about logistic is described in the organizers manual
If you plan to sell things, you can use PayPal Here. In that case, don't forget to add a note on your booth saying you accept cards.
You need to download the PayPal Here app available here. Once it's done, log in with the main Django Girls' Paypal account and connect the card reader to the app.
Note: If you can't find the app in your app store, it means your phone's location is set in a country where PayPal Here is not available. If you borrow someone's phone, don't forget to sign out at the end of the day ;)
If someone is trying to pay but the card reader hangs without timing out, it means that you need to do something in the app. The "action required" window will be hidden by the "choice of payment" window: you'll need to close it.