Okay, So once You've signed up to one of the networks(Picture Below), which network may depend on the gift you want, To Get Your Free Gift You'll Have To complete one of the offers on the page you'll get to once you sign in.
Now, there are many offers to chose from, free dvd rentals with lovefilm, make a £5 bet with coral or even sign up to bt home broadband or RAC Breakdown Cover.
Personally, I would say the Coral or gala bingo offers are the easiest, you deposit £5 and either play or bet through it, Simple
However, some people who don't want to spend anything, or who aren't 18 or over and don't have a parent who wants to do such a thing, might be tempted with one of the free offers, Either get accepted for American express credit card or a GM card, completely for free.
Or there's always The lovefilm free 14 day trail, simply sign up, revive at least 1 dvd, send it back, and cancel the offer, so long as you do it within the 14 day free trail period, it's completely free! It can sometimes take up to 28 days to credit, which can be a while to wait for your free gift.
Personally I'd Do The Coral, But The Choice Is Yours!