To 'Add a Item to a Menu', from the store dashboard go to:
> Appearance > Menus

From within this page at the top, click the drop-down box next to where it says 'Select a menu to edit' and find the menu you'd like to edit, then click 'Select' to edit a specific menu on your store:

To add a specific 'page', 'post', 'custom link' and/or 'blog category', open up their corresponding drop-down on the left hand side:

To add a 'page', 'post' and/or 'blog category', simply check the tick-boxes of the ones you'd like and click 'Add to Menu':

To add a 'custom link', input the url (for example: https://facebook.com/myfacebookpage) and apply a 'link text' (for example: 'My Facebook Page') into their corresponding fields so your customers know what they're clicking on, then click 'Add to Menu':

Your 'pages', 'posts', 'custom links' and/or 'blog categories' will then appear at the bottom of the 'Menu structure' over to the right hand side:

To rearrange the menus contents, simply drag and drop a menu item wherever you like:

To delete a menu item, click the arrow to right of it and then click 'Remove':

(*Once you're satisfied with the arrangement, don't forget to click 'Save Menu' at the top right*)