Create an index.js file in your project directory.

Add this code to set up Firebase:
```javascript
const admin = require('firebase-admin');
const serviceAccount = require('./path/to/your/serviceAccountKey.json');

admin.initializeApp({
credential: admin.credential.cert(serviceAccount)
});

// Example usage:
admin.firestore().collection('test').get()
.then(snapshot => {
snapshot.forEach(doc => {
console.log(doc.id, '=>', doc.data());
});
})
.catch(error => {
console.error("Error fetching documents: ", error);
});
```

Install express:
```bash
npm install express
```

Update index.js to create an Express server:
```javascript
const express = require('express');
const app = express();
const port = 3000;

app.get('/data', async (req, res) => {
try {
const snapshot = await admin.firestore().collection('test').get();
const data = snapshot.docs.map(doc => doc.data());
res.json(data);
} catch (error) {
res.status(500).send("Error fetching data");
}
});

app.listen(port, () => {
console.log(Server is running on http://localhost:${port});
});
```