Hi Lizzie,
Typically the menu navigation bar will wrap to another line after a certain amount of characters on most themes.
If you paid for the theme, then you will probably be better serve to visit the developer of the theme and post your question there or visit their forum if they have one.
If this is a free theme then you will likely not be able to change or modify it to the way you want without CSS knowledge.
One thing you can consider is to shorten the names of your tabs. Examples: "Contact Us" to "Help" for six space reduction. "New Arrivals" to "New Items" for another three space reduction and a total of nine space reduction which could be just enough to keep your menu items on one line.
I hope this helps you or give you ideas.