When they ask you which packages you want on the ordering screen - they call them mixes, select the ones that you want to watch. Ignore the childrens, music and style ones and you should be alright. Then you'll get the choice of adding Premium Mixes, Sports of Movies - choose the movies and you're set.
Look on their website, it's easy to work out and it will tell you how much each month will cost plus your one-off set-up fee. It starts from around �20 a month and goes up to whatever you want to pay really, I give them around �80 a month, but that covers my line rental, broadband, phone, two boxes plus the extra HD subscription
If you have any friends with Sky, speak to them first, they may well have an offer whereby signing up a friend will get them some M&S vouchers and you a free Sky+ box (normally �100). if you don't have a look through some of the weekend magazines in papers, there's usually an advert in there somewhere as well