I have HTCF Wildfire A3333 phone. I know it is old, it has Android version 2.2.1. How can I download Whatsapp on it (which version of Whatsapp)
WhatsApp Messenger supports Android 2.1+, you can just install it from the Play Store.
Be aware that with the HTC Wildfire's 384MB of RAM and 512MB of storage space, you may not be able to download, install, or properly run it though. I owned an HTC Aria with similar specs, pretty much after basic updates were applied to Google services, there was no space for anything else to be installed, and that was a couple years ago. It still shows it's compatible with that device, but when I try to install it fails due to insufficient storage.