You'll need a good seed mix,which is usually best bought from the bulk bins.The bagged or boxed stuff is often more expensive and not as fresh.You'll also want to offer treats such as millet seed on the stem.It's good for them and keeps them busy.Small amounts of fruits and veggies are also good.If the bird is not used to it though,it may take awhile before they eat it.Try offering the fresh fruit/veggies first,and save the seed mix for later in the day.Be patient and keep offering it,eventually they should try it.
Apples are a special favorite for most birds...but feed sparingly so as not to cause watery droppings.A healthy bird should have mostly firm droppings with a mix of bright green and white.Be sure to provide fresh seed every day.Birds have been known to starve because the owner didn't realise that what was left in the dish was just seed hulls.The dish may appear half full but empty it and refill daily.
A vitamin/mineral supplement may be helpful.Also provide cuttle bone and toys that re appropriate for small birds.These can be expensive and the bird will probably tear them up...but that's what they are for
Sometimes the hardest thing is to find a vet who can treat your birds.Many vets are not trained in avian care.Hopefully your birds will never be sick or injured...but it's best to find a vet before you have an emergency.It's also good to have one who can clip the wings if you are not able to do this yourself.It may seem fun to have a bird flying about and it may seem cruel to clip the wings.But birds can easily be lost, injured or even killed if they are not clipped.The wings need to be clipped regularly as the flight feathers are replaced.Bear in mind that a bird with clipped wings may still be able to gain some altitude,though the flight will be clumsy.It's best always to keep the bird under very close supervision when they are out of the cage.
Also note that while cats pose a threat to birds,dogs may also go after them.Some breeds are more inclined to do this than others.And ferrets are especially dangerous to birds.Unlike cats and dogs,a ferret can not(in my experience)be trained to leave the bird alone.Again...close supervision is a key to keeping your birds safe.Never leave them untended with other animals around.
Hope that helps.I'm sure others will have good advice as well.Birds can be very charming companions and will provide you with years of happiness if they are well cared for.A tame and cuddly bird is a special pleasure.If you get birds that were hand raised they can be quite affectionate and sweet.Best of luck to you
Koda