The RSPB has a great page on the do’s and dont’s of feeding your garden birds. Although feeding birds can be an all year round activity, it is of course particularly valuable over winter when food sources are scarce.
The article covers everything from where and when to feed, how to set up your bird feeders for maximum benefit to wildlife, as well as what not to feed birds, and this might not be what you were expecting if you are used to leaving out bread for them (especially in spring), and how to feed them in a way that doesn’t attract other visitors such as rats, which have been known to predate on wild birds.