Once you are logged into the admin area navigate to "Media" (located on the left hand side). Upload an image/document (if it is not already there) by clicking "Add New" and following the on-screen instructions. Once you have an item in the media library that you want to link to, click on that item to show more information.
On the right hand side of the "Attachment Details" screen you will see a field labelled "URL" you need to highlight and copy (ctrl+c) this so we can use it in the next step.
To create a hyperlink to that media item within a page/post you will need to highlight the text you want to become the link, then press the "insert/edit link" button on the editor.
Add the URL you copied from the right hand side of the "Attachment Details" screen into the filed and provide an optional title for the link
Press the blue "Add Link" button and publish/update your page/post to complete this process.