The text app on your Android mobile might not be the same as the one on mine (which is an Alcatel model) but I also see a text displayed with the name of the sender at the top and a left-pointing arrow so, hopefully, I might be able to help you.
Firstly, that arrow takes you back to the full list of texts stored on your phone. (It will probably only work if you've got to the text by going to the 'text message' icon first, to view that list. It will probably do nothing at all if you've gone direct to a text that you've just received).
However, at the bottom of my screen there's a box labelled 'Type text message'. (I'm hoping that you've got something similar, otherwise this post is probably rather pointless). Next to that box is an icon that looks like paper dart.
If you tap in the box I've just mentioned you'll bring up a keyboard, which allows you to type your message. (There will be buttons on the keyboard which enable you to turn capitals on or off, or to switch to numeric/special characters). As you type, you'll probably see suggested words across the top of the keyboard. (e.g. if I type 'crossw' it suggests 'crossword'). When you see the word that you actually want appear, simply tap on it.
When you've completed your text message, tap the 'paper dart' to send it.