There are all kinds of food and food mixes you can buy for birds, but there are a lot of substitutes as well. Poultry food, dog food, cat food, and lots of other foodstuffs can be used. But storebought wild bird seed is probably the safest,

Foods

Black-oil sunflower seed: high in fat so it provides good energy; seeds are small and thin-shelled enough for small birds to open
White Proso Millet: high in protein content
Peanuts: offer in tube-shaped metal mesh feeders designed for peanuts; use a feeder with smaller openings for peanut hearts
Suet cakes: commercially made suet cakes fit the standard-size suet feeder (you can even find vegetarian options)
Nyjer seed (for goldfinches): use a tube feeder with tiny holes to keep the seeds from spilling out
Cracked corn: medium-sized cracked corn; fine corn will quickly turn to mush and coarse is too large for small-beaked birds.

Fruit: many birds eat fruit like bananas or orange slices, but this would mainly be in the spring when tropical-wintering birds arrive.

Nectar: although mainly for hummingbirds, orioles and even woodpeckers may enjoy this sugary treat.

Meal worms: live or dried are attractive to some birds that don't usually eat seeds, like robins and bluebirds.

No-No Foods

Bread (fresh or stale): provides no real nutritional value for birds, and moldy bread can harm birds
Chocolate: toxic to birds, just as it is to dogs and cats (it contains theobromine)
Table scraps: some may not be safe or healthy for birds, and most table scraps will attract mice or rats

Add Water

To get more birds to your feeder, provide water — especially during drought or when the temperature stays below freezing for several days. 

Add a bird bath to your yard. Replace the water every day or two to keep the water fresh and clean. Birds often leave feces or feathers in the water, which can grow bacteria that can spread to other birds. Wash the bird bath every week or two with a weak vinegar-water solution (nine parts water to one part vinegar). To keep water from freezing in winter, you can get a heater to place in the bird bath or find affordable heated bird baths.

Maintenance

Wash feeders regularly to prevent the spread of diseases between birds.

Hummingbird feeders should be washed every week or two to keep mold and bacteria from building up. During hot, humid summer weeks, wash feeders every 2–3 days — and replace the sugar water just as frequently. It is especially important to check the small openings through which the hummingbirds drink to make sure there is no black mold.

Don't let seeds accumulate on the ground. Although some birds might feed there, a pile of seeds is attractive to mice, rats, squirrels, and even bears. And you'll be growing a lot of weeds in the spring.