diff --git a/config.py b/config.py
new file mode 100644
index 0000000..e69de29
diff --git a/core/__init__.py b/core/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/core/item.py b/core/item.py
new file mode 100644
index 0000000..eeedcc9
--- /dev/null
+++ b/core/item.py
@@ -0,0 +1,12 @@
+class Item:
+    def __init__(self, id, name, price) -> None:
+        self.id = id
+        self.name = name
+        self.price = float(price)
+
+    def to_json(self):
+        return {"id": self.id, "name": self.name, "price": self.price}
+
+    @staticmethod
+    def from_json(data):
+        return Item(data["id"], data["name"], data["price"])
diff --git a/core/transaction.py b/core/transaction.py
new file mode 100644
index 0000000..d9ceeef
--- /dev/null
+++ b/core/transaction.py
@@ -0,0 +1,29 @@
+from datetime import datetime
+
+
+class Transaction:
+    def __init__(self, id, user_id, item_id, amount, timestamp=None) -> None:
+        self.id = id
+        self.user_id = user_id
+        self.item_id = item_id
+        self.amount = float(amount)
+        self.timestamp = timestamp or datetime.now().isoformat()
+
+    def to_json(self):
+        return {
+            "id": self.id,
+            "user_id": self.user_id,
+            "item_id": self.item_id,
+            "amount": self.amount,
+            "timestamp": self.timestamp,
+        }
+
+    @staticmethod
+    def from_json(data):
+        return Transaction(
+            data["id"],
+            data["user_id"],
+            data["item_id"],
+            data["amount"],
+            data["timestamp"],
+        )
diff --git a/core/user.py b/core/user.py
new file mode 100644
index 0000000..60f21c6
--- /dev/null
+++ b/core/user.py
@@ -0,0 +1,14 @@
+class User:
+    def __init__(self, id, name) -> None:
+        self.id = id
+        self.name = name
+        self.debt = 0.0
+
+    def to_json(self):
+        return {"id": self.id, "name": self.name, "debt": self.debt}
+
+    @staticmethod
+    def from_json(data):
+        user = User(data["id"], data["name"])
+        user.debt = data["debt"]
+        return user
diff --git a/data/__init__.py b/data/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/main.py b/main.py
new file mode 100644
index 0000000..e69de29
diff --git a/storage/__init__.py b/storage/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/ui/__init__.py b/ui/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/utils/__init__.py b/utils/__init__.py
new file mode 100644
index 0000000..e69de29