Part of the text that NPC says just goes to the side of the screen. How do I make it all appear?

This depends on whether that text box has a Face Graphic or not. If it does not, make sure the text does not go past the second little arrow.
If you do have a Face Graphic for that text, then make sure the text does not exceed the first little arrow.
You can reduce the length of the text by pressing 'Enter' to make it move down to the second line.

Example

Thanks.

That threw me off the first time, too. But it's actually pretty easy. When you input text, there's a Preview button. Press that, and you'll be able to tell if it's going to run off. Then you can determine how you need to break it up. For instance... instead of "My ROOM is fun too! But I guess you can grab one and bring it with you." Break it up like this:
"My ROOM is fun too!
But I guess you can grab one and bring it
with you." Then it looks nice and clean. Just play around a bit for the best look.

Thanks for the info. But that means I'll go to a few characters who spoke loads and edit every text I made.

It can be a pain at times, but a good developer would be willing to edit out and change lines in their dialogue if it help make the game be more of a success for players.

Sounds like information can be cut-off if you don't do this, so it's a must if you want your game to be good.

As the developer just remind yourself to constantly look at ways to improve the game, it's worth the effort if it keeps helping improve the game. Just don't get overwhelmed and confused with it, break it into simple steps and polish the areas that have bugs, problems or players simply dislike for other reasons.

After all if a developer is not willing to do this, then what is the point of making a game players can't enjoy?

If it's for practice. How are you practicing if you don't either make the changes to your current game? Or cancel it and apply these changes to the new project? And I'd only recommend cancelling a project if it's either going no-where or really better starting off from scratch. Not simply bug fixing, like text cut-off screen.