Ants will follow each other in a trail, using the scent of pheromones to accomplish this. You can follow the ant trail to find out where they are coming from and where they are going. These paths are what you will want to treat.
The worker ants are the ones you will see most often. These workers will bring food back to the colony. They cannot actually eat the food themselves. Both the workers and the queen will eat a liquid by-product that the larvae produce when the food is fed to them.
Baits are the most effective and safest way to kill ants but before you buy bait, you will want to see what the ants are eating. Most ants will eat sugar or protein based foods. Leave a little food out for the ants, sugar and protein based, so you can find out what they are eating.
Once you know what your ants are eating, you can buy bait with the proper protein or sugar base. The worker ants will carry the bait back to the colony for the larvae. When the workers and the queen eat the by-product of the larvae, you will be reaching all stages of the ant at once.
NiBan is bait that is mixed with Boric Acid. Boric Acid is about as safe as it gets. It has only half the toxicity of table salt. If you have very tiny ants you will want liquid bait. The liquid bait will consist of a simply syrup such as Taro with Boric Acid dissolved into it. You can use this the same way you would use granules.
If you want to hit them twice, you can use a safe dust or spray from EcoSMART. EcoSMART makes products that are so safe, some of them are exempt from EPA regulations.
Use one or both of these safe, natural methods and your yard will soon be ant and chemical free.