How to iMessage which people that don’t have an iPhone

If you also have an iPhone on the same Apple ID, the iPhone can relay messages between the iMessage system and the SMS/MMS system.

On the iPhone, visit Settings > Messages > Text Message Forwarding and enable the entry for your iPad.

Thankon wrote:

How do I message from my iPad to people that don’t have an iPhone

You can't unless you have an iPhone using the same Apple ID.

You can if you also have an iPhone, read here => How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Do you own an iPhone?