Gedit
Gedit is the official text editor of the GNOME desktop but also works in other desktop environments including XUbuntu. It can only be run in graphical mode.
To start gedit, open a terminal window and type
gedit filename &
or select Applications -> Accessories -> gedit from the XUbuntu menus.
Gedit comes with well documented help. Open gedit and select Help (from the top right hand side) -> Contents.
You may find when you start gedit that there are no tools listed under the Tools menu. With the Maths install you need to select the tools you want. Open the help pages and learn how to load gedit plugins (Edit->Preferences->Plugins - select the plugins you want). Once loaded, you can select the appropiate tool when you need it. One of the tools is a spellchecker.
You can use google to search for gedit tutorials.
There is more information at https://wiki.gnome.org/Apps/Gedit.