For a standard text message, 153 characters will use one credit. The standard font is Ariel, size 13. You'll be able to see if you're using the standard font when you're writing your SMS as you'll see how many characters you've used and how many per credit you have.
If you change the font or text size the SMS will be considered to be using a none standard character set and you'll only have 67 characters per credit. You'll be able to see this as well:
If you accidentally change the font or size and you want to go back to the original settings you'll need to click on Discard Draft at the bottom of the page to remove the formatting.
Please note: Discarding your draft will remove all of the text you've written thus far. If you want to retain this text you'll need to select it all and copy it (CTRL+C) then after you discard your draft you'll need to use Ctrl+Shift+V to paste the text without the none standard formatting.
Is the SMS signature included in credits used?
Yes, the SMS signature is included in the number of credits used. You can see how many characters your signature has when you're writing your SMS.