To set up and use App Lock on your Samsung Galaxy M15, you can use the built-in "Secure Folder" feature or download a third-party app from the Google Play Store. Here's how to do both:

Using Secure Folder:

1. **Set Up Secure Folder**:
Go to **Settings**.
Tap on **Biometrics and security**.
Select **Secure Folder**.
Follow the on-screen instructions to set up your Secure Folder. You'll need to sign in with your Samsung account if you haven't already.

2. **Add Apps to Secure Folder**:
Open the *Secure Folder* app.
Tap on **Add apps**.
Select the apps you want to lock and add them to the Secure Folder.

3. **Access Locked Apps**:
To access the apps, open the *Secure Folder* and authenticate (e.g., fingerprint, PIN, or pattern).

Using Third-Party Apps:

1. **Download an App Lock App**:
Open the **Google Play Store**.
Search for *App Lock**. Some popular options are **AppLock* by DoMobile Lab, **Norton App Lock**, or **Smart AppLock**.
Download and install your preferred app.

2. **Set Up the App Lock**:
Open the app you downloaded.
Follow the on-screen instructions to set it up. This usually involves setting a PIN, pattern, or password.
Grant the necessary permissions to the app.

3. **Lock Apps**:
In the app's settings, select the apps you want to lock.
Configure any additional settings according to your preferences.

Both methods offer a secure way to protect your apps. The Secure Folder provides a more integrated solution, while third-party apps offer more customization options. Choose the method that best fits your needs.