Check your phone for malware. Review the installed apps (Settings > Apps) and uninstall those you don't recognize. You can also download Malwarebytes to scan your phone. Once you're sure that your phone is clean, follow the next steps.
Uninstall WhatsApp and restart your phone. Next, download the official WhatsApp app from Google Play and activate your account - you may have to wait 7 days before you can activate it without the 2-step verification.
When your account is activated and accessible on your phone, tap the 3 dots and select Linked devices. Make sure there're no devices linked, except your phone. Then, to secure your account, enable the two-step verification by going to your WhatsApp settings > Account > Two-step verification. This will add an extra-security for your account and prevent unauthorized access.