First thing I would suggest would be to decide what you want
To use the area for.
Following on from that divide the overall area into sections, you can do this on paper to begin with and then use string to line out garden.
Beds and boarders are easy to cut out.
I've planned and planted 4 gardens of verging styles, anything from Chinease maintenance free to cottage garden.
There is an amazing garden fate coming up in glanmire in May and they have plants at really cheap prices and all mature and non garden centre