Hello, and welcome to GrapheneOS.
There's a Messaging app which comes installed on your phone. In the first instance, you can use that.
From the home screen, swipe up from the middle of the screen to show the app drawer. In your app drawer, you'll find a list of all the apps you have installed. Scroll down to 'M' and tap on Messaging to open it.
Now you should have the Messaging app open. You'll see a list of any texts you have sent or received there. Tap the green circle with a white plus. It's in the bottom right hand corner of your screen.
You can then scroll through your contacts until you find the person you want to text. Tap their name, and you'll now see the screen where you can start to write them a message.
If you want to text multiple people, tap the cursor next to the name of the first contact. It should be shown at the top of your screen. That will open your keyboard. You can then start typing the name of the second person, and it will show you a drop down of your contacts. Then tap their name to add.
Once you've added all the recipients, you can write and finally send the contents of the message.
Hope that helps.