Have you tried the BBC gardening website.... there is a garden design section (a la Titchmarsh) that takes you through some of the basics;
Once target=_blank>http://www.bbc.co.uk/gardening/htbg/index.shtml
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you've got a design, I'd probably start by using a weedkiller on the garden (if the lawn's bad then that as well - it'll be easier to lay a new one!), leave it a couple of weeks to die down and then start from there (use a systemic weedkiller that attacks the weed but breaks down in soil - I use 'Roundup') But, if you have any good specimen plants that you'd like to keep, be careful!!