In MacOS, if you use the iMessage app, it will automatically determine whether to send a text message or iMessage. To send a message, use the iMessage app. This icon, on the menu bar, looks like the normal texting app on the iPhone. If you want to send a text message, for example to contact someone who is using an Android phone, just start a conversation as you normally would to someone with an iPhone. In your phone (if you have an iPhone), in the iMessage settings, let your mac send text messages through your phone number. After this, you should be able to send iMessages and test messages through your mac.
how do i text message off my computer (mac book pro) to text people?
In a similar situation I use to have iMessage enabled on my iPhone and iPad, however I did find that when I was out, my iPad would pick up the texts. The texts would only show on my iPhone when I got home and connected to Wi-Fi.
This maybe as I didn't have a data package on my iPhone....