Created first panel and added it to window

src/de/mp/jgame2d/core/Boot.java

@@ -10,6 +10,11 @@ public class Boot {
         window.setResizable(false);
         window.setTitle("JGame2D");
 
+        GamePanel gamePanel = new GamePanel();
+        window.add(gamePanel);
+
+        window.pack();
+
         window.setLocationRelativeTo(null);
         window.setVisible(true);
     }

src/de/mp/jgame2d/core/GamePanel.java

@@ -1,10 +1,25 @@
 package de.mp.jgame2d.core;
 
 import javax.swing.*;
+import java.awt.*;
 
 public class GamePanel extends JPanel {
 
     // SCREEN SETTINGS
     final int originalTileSize = 16; //16x16 tile
+    final int scale = 3;
+
+    final int tileSize = originalTileSize * scale; //48x48 tile
+    // 4:3 ratio
+    final int maxScreenCol = 16;
+    final int maxScreenRow = 12;
+    final int screenWidth = tileSize * maxScreenCol; // 768 pixels
+    final int screenHeight = tileSize * maxScreenRow; // 576 pixels
+
+    public GamePanel() {
+        this.setPreferredSize(new Dimension(screenWidth, screenHeight));
+        this.setBackground(Color.black);
+        this.setDoubleBuffered(true);
+    }
 
 }