Since your Nokia X doesn't have access to the Google Play Store, you will need to download and install Whatsapp manually.

1. Enable side-loading of apps on your Nokia X by navigating to Settings -> Security and enable the Unknown Sources option.
2. Download the Watsapp APK directly from their site to your computer.
3. Transfer the apk file you downloaded onto your Nokia X by connecting it via USB cable and using a file manager, send it via Bluetooth, or copy it to an microSD card and insert it into the phone.
4. Run the pre-installed ASTRO file manager and locate the file using the search option.
5. Tap the file to run it, which will start the installation process.
6. Follow the on-screen instructions to complete the installation.

You should now be able to launch Whatsapp like any other installed app on your Nokia X phone.

Since your Nokia X doesn't have access to the Google Play Store, you will need to download and install Whatsapp manually.

1. Enable side-loading of apps on your Nokia X by navigating to Settings -> Security and enable the Unknown Sources option.
2. Download the Watsapp APK directly from their site to your computer.
3. Transfer the apk file you downloaded onto your Nokia X by connecting it via USB cable and using a file manager, send it via Bluetooth, or copy it to an microSD card and insert it into the phone.
4. Run the pre-installed ASTRO file manager and locate the file using the search option.
5. Tap the file to run it, which will start the installation process.
6. Follow the on-screen instructions to complete the installation.
7. Important step: Modify your phone’s date to (or just prior) 2nd of August of 2014 (don’t worry you will change it back to the preset date afterwards.)
8. Launch Whatsapp like any other installed app on your Nokia X phone, and complete the initial set-up.
9. At this point you will most likely get a message saying that this application is "not supported" on your phone.
10. Exit WhatsApp when it says that your device is not supported back to the homescreen.
11. Uninstall the official Whatsapp application by long-tapping on the WhatsApp icon and tapping on the red cross.
12. Now download WhatsApp Plus from here and install it using the same method as the official app (steps 3 through 6 of this guide.)
13. Open Whatsapp Plus app and go through configuration steps. Make sure to use the same phone number as you used in the original WhatsApp, otherwise it will not work.
14. You can now revert the system's date back to the present date and still enjoy WhatsApp.

You can now use Whatsapp plus to connect to your Whatsapp contacts.

Sources: 1, 2.