Every year in the UK, 14th -21 February is National Nest Box Week. Gardeners and bird-lovers across the country are encouraged to put up nest boxes for their feathered friends! If you already have nest boxes, now is a good time to clean them ready for their new occupants this breeding season (if you didn't already clean them at the end of the last breeding season).
The British Bird Lovers website has a comprehensive section on bird boxes, which you can browse here. While the RSPB has a useful article, with links here.
Once you've fitted a nest box, you can join in the British Trust for Ornithology's Nest Box Challenge. This encourages people to monitor which birds use their nest boxes and how successful they are. Monitoring should of course be done without disturbing the birds, either by fitting a small camera into the nest box before putting it up or by limiting observations to watching the birds coming and going. You should not open the nest box up during the breeding season.
If you enjoy watching birds visiting your garden then it makes great sense to offer them an easy home. They will reward you with entertainment and many species will also help keep down the number of insect pests in your garden.