How can I send a message to an Android from my iPad

Unless you also have an iPhone, your iPad cannot send or receive SMS or MMS messages to phone numbers - as these are Cellular Carrier features. Without an iPhone, your iPad is limited to sending/receiving messages to other Apple devices via the Apple-proprietary Messages App.

Native iMessages are directed to Apple recipients by their registered email addresses - and not by their Cellular phone numbers. The Cellular number associated with your iPad’s Data-SIM is only used for account identification and billing purposes.

To use SMS/MMS messaging from the iPad Messages App, your iPhone must be logged-in to iCloud with the same AppleID - and message forwarding to your iPad must be enabled on the iPhone:

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac – Apple Support

More information about Continuity and the relevant Cellular services can be found here:

Continuity

https://support.apple.com/en-gb/HT204681

SMS/MMS Messaging

https://support.apple.com/en-us/HT208386

WiFi Calling

https://support.apple.com/en-gb/HT203032

When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.

I hope this is helpful in clarifying your iPads Messaging features.

Your iPad is not a cellular telephone. Whether you have a cellular iPad or not, it cannot send or receive SMS text messages.

If you had an iPhone, you could send/receive SMS messages.