If your garden is sufficient in size, clear a small area at the bottom and sow it with a wildflower/grass seed mix. This needs to be cut only once a year in late summer after the wildflowers have seeded. Also make a pond, it can be quite small but make sure that there is one point which is very gently sloping. This will not only allow creatures to drink from the pond but will also allow them to get out if they fall in. Any garden with water will attract wildlife. One other thing is to create a small wood pile again, if size permits.
In general gardens which are not too tidy are better for wildlife and many species will benefit if you can leave a nettle or bramble patch or simply allow rough grass to grow alongside a hedge. Native plants and flowers are good for invertebrates and flowers which produce nectar will attract bees and butterflies whereas many hybrid varieties whilst having showy flowers are nectarless. Wildlife needs breeding habitat, shelter, food and water – for example providing nest boxes, feeders and water for drinking and bathing will ensure your garden becomes a mini nature-reserve for birds.