I want to turn some words on my website into a link which will link to another tab. How do I go about that please

If you mean you want the page linked to to open in a new tab in the browser, this is not possible. You can force a linked page to open in a new window:

EDIT: This page suggests a JavaScript which will allegedly open a page in a new tab; I haven't tried it so I don't know whether it works, and I doubt you will be able to do this in iWeb or any program where you can't edit the coding.

By another tab I'm assuming one of your other pages in the website or is it an external webpage?

In either case select just the word itself in the text box and go to the Inspector/Link/Hyperlink pane and select either One of My Pages or An External Page. If you select An External Page you will need to know the URL to that page.

You can then go to the Format pane and set the various link colors you'd like to use for that one link.

OT

You should know by now that everything is possible in iWeb.

http://www.wyodor.net/_Demo/OpenInTab/

Interesting; didn't work in Safari until I changed the relevant preference back to the default. Didn't work in Firefox - not sure there's a prefs setting will deal with this. Worked straight off in Chrome.