When it's a text hyperlink you double click the text and a new window appears. At the top of this window you see Href:. Here you fill in the location of the page you want to link to. When this is a page in the same folder as all the other pages of your site the name with the extension is enough. For example nextpage.htm when it is a page of a totally different website you need something like http://www.otherwebsite.com/otherpage.htm Then you choose if you want to open this new page in a new window or the same one. This you can choose in the top right corner where it says Target: _blank opens a new window _self uses the current window.
Same goes for a image hyperlink. Double click. In the Href field fill in the location and in the target field fill in _self or _blank.