Hi Karizu welcome to WAB..
I used an on-line calculator it works the pond liner size out for you..
I went to Buckingham garden centre for mine they do on-line delivery but I live close enough so I picked it up and the underlay..
Pond Liners Direct: Pond Liners, Pond Pumps, Pond Filters: UK
I made a shallow edge about a ft deep all round then poured gravel on but slant it back as the gravel will fall into the bottom otherwise. Then everything has has easy access in and out also the blue tits will bath as its not too deep for them.
I put a turf edge on one side and let the grass grow newts,toads and frogs go in between the turf and the liner.
I planned all mine out on paper first so I knew in my head what I wanted it look like...
I looked to see how Alan Tichmarsh did his on how to be a gardener..
I got some good tips having never dug a pond before.
BBC Gardening - How to be a gardener Part Two
Try to post some pics when you start any problems you face there will be someone more knowledgeable who will be able to help..