There are a lot of ways to do this. I don't use Astro, but I know this can be done with Estrongs file explorer, for example. Open it, then menu->new->File. Just call it whatever you want, and it will automatically be a text file. When you tap on it, you'll have the option of editing it as a text file. I'm guessing the same can be done in Astro somehow, but it's easy in ES. Another option is to make your text file on your computer, then just copy it to your card. Depending on how much text you're typing, that might be easier. Just a quick warning on that, though, I recommend you use something like textpad (google it) rather than notepad. Notepad will put in junk characters that Android (linux) won't be able to understand. Depending on what this text file is for, it might cause problems.
That being said, are you trying to make a text file for use with setcpu? There are only a couple of reasons for doing this. Are you sure you need to?