You can buy unlimited supplies of affordable, healthy food for your Neopet at the General Store in Neopia Central! This store is always stocked so you will always be able to find a tasty treat for your pet! To visit the General Store, go here.
To feed your Neopet with an item you already have, visit your inventory. You can find your inventory through the Shops link or by clicking the number of Neopoints you have at the top right corner of each screen. Choose the item you want to feed to your pet and a new window will open up with a drop down menu. Choose "Feed to [your pet's name]" and then "Submit". The window will then change to your pet's picture and you will know if you were successful in feeding him or her!
If you have less than 2000 Neopoints then you can take your pets to the Soup Kitchen. The Soup Faerie will feed them a nutritious broth for free. The Soup Kitchen can be found here.