Lots of great advice for you on here. I would leave the garden for a year - that way you'll begin to understand what microclimates occur which will in turn lead you to decisions about the eventual design. It also gives you time to mull over different ideas rather than rushing into anything. You can always set up some bird feeders to tide you over for a year.
If you can't wait then I'd would definitely recommend some preliminary reading: Try Ken Thompson's book 'No Nettles Required' which debunks many of the myths about wildlife gardening. It's a very accessible book ( I read it in a couple of days) and its findings are based on the Biodiversity in Urban Gardens project that Thompson led at Sheffield University. I'd recommend it to anyone. I'd also really recommend read Rain Gardens by Nigel Dunnett and Andy Clayden also at Sheffield Uni which focuses on water but has masses of great ideas relevant to wildlife ... and no I'm not connected with Sheffield Uni in anyway
A good design book is probably worth a look too. Try one of John Brookes books.
Also, get yourself a tracing pad, map out the garden on one sheet then use overlays to design your garden - paths on one layer, trees on another and so on. It's important that the garden works for you as well as for wildlife. Don't worry if you don't have much to spend. If you are prepared to put in the work you can create something you'll be really proud of by a little planning and some scrounging of plants and materials from friends and family. There are also lots of cheaper and environmentally friendlier alternatives to some of the materials that you'll see in garden design books. Many nurseries sell trees in 'whip' form which are cheap as chips (literally) and will catch up with an expensive garden centre tree in 2 or 3 years.
Good luck with it and keep us informed on the forum