How do i start to write a book?

[Star]

hey, im 15 and want to write a book just for me, a sort of imaginary world for my eyes only. I live in britain and its totally boring so i need a kind of escape. In my book i want too write and draw so im unsure wether to get a lined of plain paged book. Also where to buy one from, A4 size and loads of pages. How could i make my world come to life. I dont want it as a diary i want it to be completly different to my normal life. i will make up characters and creatures and draw them in the book. lined or plain? how to start? where to buy? thanks xx