before you buy any plants find out what type of soil you have, also which direction your garden is facing ie north, south, west etc. Having this knowledge will help you when you go to the garden centre to choose. Have some idea how you would like the garden to look. Plant two or three large items first, choosing carefully where to plant them, these will give the garden some shape. Think about decking or a patio as these should be done early on in the development. Its very easy then to read the information on the plant labelling to fill the spaces in between.
Enjoy doing it, it will give you hours of pleasure.