Right Matt, when you're on the page you want to link to, Drag your mouse along the address(e.g. http://www.google.com) in the website address bar at the top of the browser. If you're using a laptop double-click on the address bar. In both instances the address of the page should be highlighted. Right-click on the highlighted address and choose the copy option.
Now, back into this site and when you're posting a reply or a new topic you'll see a small little icon in the toolbar above the writing which looks like a little planet earth with a chain link in front of it. What you do is for example type the words 'Link here'. Now highlight the word 'here' by dragging the mouse over it or by double clicking on the word. With the word highlighted now click on the little earth and chain link icon. It'll ask you what link you want to put in place of that word. Rightclick and paste the address you had saved earlier into the address box provided. Then press done. Where you have typed 'Link Here' the word here now will give a link to the website that you wish to link to. 'Link Here' for example