Normally that will happen with a template and not a signature. So you have specified italics at least in the first line of the template. But it can happen with a signature if there is some formatting that is affecting the previous few lines where you will type. In both cases you will need to edit the template or signature and remove the italics then see if there is any difference.
But it can also happen if you have specified the font in Menu > Settings > Mail > Compose > Preferred style. By default it is Segoe UI Regular, but if you changed that to Italic, that will be the cause.