Notice that I'll be using lots and lots of shortkeys in this tutorial; in the future it will make your workflow way shorter.
1. Open a new document for your illustration. (Alt+ N) and make it square. I've made mine 200mm by 200mm.
2. Create a perfectly shaped circle on your artboard. To do so press L on your Keyboard and then Shift+Alt+Drag until you're circle fills half of the artboard.
3. Press Shift+C on your keyboard to select a Convert Anchor point.
4. Press A on your keyboard, choose both vertical points of the circle and hold the Down arrow on your keyboard you have a nicely shaped heart end.
6. Press A, then the Alt key and raise the handles to almost vertical position so your heart gets a nice curvy shape. The basic shape is ready.
7. Now let’s add so some color to it. Choose a dark burgundy color from your color picker.
8. Go to Effect - Stylize - Inner glow.