You have to make some css modifications. First of all the width of the UL element in the twenty-twelve theme menu, is set to 100%. Override that by
width: auto;
Lastly, change add a new property for the same element.
float: left;
This way all the menus will be arranged side by side there is enough space on the screen. Other wise they will be arranged one below the other.
i inspected the elements; which file would i need to find to make these changes? Is it:
wp-content / themes / twentytwelve / style.css?