If you only wish to share one file, find the file that you want to share. Click on the Three Dots next to the filename (circled below). In the menu that appears, select 'Share'

In the next screen, add the email addresses of the people you wish to share the file with and any message you want to include. 

You can then click on 'Send'. There are also options to use Outlook 2016 and to get a link to the file to use in a document. Folders can also be shared using this method.

To share multiple files and folders, first select the items you want to share by clicking on the roundel next to the filename, this appears when you hover your mouse over the document:

Once selected, click on 'Share' at the top of the page:

You will then get the same sharing options as sharing a single file or folder: