What a great blank canvas you have and a fantastic opportunity to create something very special!
I started a brand new garden four years ago and I mention this as it was very similar to yours but I did have an idea of what I wanted to create. I wanted to create a space that was wildlife friendly but also catered for all the family including my (now) 6yr old daughter. I also wanted to use recycled or reclaimed materials as much as I could within the garden.
This makes me think that before you do anything you need to decide what you actually want out of the garden and who is going to use it?
So what function(s) does the garden have to cater for? Wildlife, Children, an outside party space, growing vegetables etc etc??
Sounds like a lot to decide but this will be key to how you start and continue to develop the garden. It also is a decision that needs to be made prior to any work in my opinion.
Once you have decided what you want the garden to do get a piece of paper and a pencil and sketch what you want in the garden and where you think it should go. If unsure of planting certain plants then post questions on here as someone will be able to advise as you go along. It's also of interest to other gardeners to see a project develop from scratch.
I am no expert by any means but I have been keeping a blog diary of my garden as I've developed it over the last four years (link below) You are welcome to have a look and see if anything is of interest or gives you ideas for your own plot. If you go into the Archived posts on the right hand side you can see how the garden looked prior to starting, which I think you will agree is in a similar condition to your patch! Some of the projects have also been posted up in a step by step guide with photographs that might also help you to see how certain projects and themes can be practically implemented?
There are many very good blogs on line and by members on here so ask about or search google as they can be a good source of information and ideas of what others are doing.
If anything is of interest on mine please feel free to drop me a personal message either on here or via my blog and I'd be more than happy to help further if I can.
So in conclusion, I suggest decide what the garden needs to deliver for you and your family. Do some research to help and then get a plan down on paper. If you want the garden to deliver several different uses you can always divide it up into different areas if you want, or develop as one whole area, the world is your oyster as they say!.. Oh and keep asking questions as there's many people willing to help where they can!
I watch with interest to see what you decide and how the garden develops!..