All pages and some modules have an editor in them called a WYSIWYG. It has a whole bunch of useful editing tools such as bold, italic, colours, fonts, bullets, tables, etc.

If you are familiar with HTML you may wish to switch this off in order to gain greater control over your formatting. You can do this by clicking on the HTML editor icon as shown below

This will switch from your standard editor to a HTML editor

Once you've finished making your changes, click Update and then Save at the top right. Remember that Joomla times out after x number of minutes (this can be set in the global configuration), so don't spend too long adding things before you click Apply at the top left to save your changes.