In article , olrac
writes
I have just moved into a house with a garden 100 x 40 foot with nothing in
it.

what should I do
Have a look at the urg FAQ called 'Making a Start' at:
http://www.nugget.demon.co.uk/MetaFAQ/startingout.html

what is the best software for designing gardens

I don't know about software, but the best hardware is a fork and spade.
--
Alan & Joan Gould - North Lincs.