Buy a feeder that will hold about a pound or two of seed. That amount doesn't have to be precise. The point is that you want to get a feeder big enough so that you don't have to refill it every day, but not so big that the seed will go moldy before the birds have a chance to eat it. For most people, that means a feeder holding a pound or two of seed, like the Droll Yankees A-6 model. Put something big and easy to clean under the feeder.
Birds are messy eaters.
They throw seed around. They scatter seed hulls, to say nothing of droppings. If you let this stuff accumulate under the feeder it will not only make a mess but also, potentially, spread disease to other birds. That's why you can buy trays that attach to bird feeders or can be hung under them. In choosing a tray, bigger is better because bigger trays catch more.