One of the easiest ways to create habitat is the log pile. This will help to provide a place for everything from spiders and beetles to hedgehogs, toads and ladybirds.
Small branches and logs can be layered in a sheltered spot, perhaps behind your shed or behind some tall plants. Try to avoid putting too close to living trees, bacteria and fungus can infect these. You want a cool, shady and maybe damp-ish location. Leave sufficient space between the logs (especially on the bottom layer) to let those creatures in.
These links give more advice with more in-depth information. Discoverwildlife.com, Wildaboutgardens.org.uk and the RSPB.
If you've no room for a log pile or no logs try these alternatives:
Insect houses (pretty spectacular huh? Some great ideas in there, but you can create or buy smaller less...over the top versions easily!)
Toad houses (Basic DIY )
Compost heaps (Great article on compost and wildlife)
Garden walls can create a habitat to bees and bugs (don't fill in those holes and cracks!)
Hedges of native plants (lets get rid of of that suburban 6' high, untrimmed box hedge. Please.)
Bird and bat houses