Adjusting Main Menu Tabs
Website styles in general are coded through CSS. 

Drupal implements that through themes. If you're using the default Bartik theme you can override the navigation item spacing with the margin property on the list items. 

It's not recommended to modify an existing theme file if you need to adjust its style. The best practice is to create a subtheme based on the original and include the modifications in the subtheme so they override the original one.  

If you're not knowledgable in themeing or CSS you can make small CSS adjustments with the help of a module like CSS Editor. This way you don't have to worry about themes or subthemes, the code will always apply. 

You can try something like: 

#main-menu-links li {
  margin-right: 20px;
}


You may have to clear your CSS cache in Drupal to see the changes, if you have the CSS aggregated under Performance.