There are a few ways to encourage hens to lay their eggs in a specific spot. 

It is usually recommended to have one nest box for every 4 or 5 hens, but at least 2 nest boxes even for a small flock. Nest boxes need to be appropriately sized for the breeds you have and easily accessible. For standard-sized breeds a 12x12x12 cube usually works well, but you may want a larger box for Jersey Giants or Brahmas. Think about the location of your nest boxes - they should be in a darker, quieter area of the coop because hens usually won't lay eggs in bright, high traffic areas. Nest boxes should be clean and comfortable for the hen. A soft nesting material like pine shavings, a nest pad, or straw can make nests more inviting, but make sure that the material stays clean.

Assuming your nest boxes are setup well, you could try adding some fake eggs to give the hens a hint. Hens like to lay where other eggs already are, so this often works. You can find plastic, ceramic, rubber, or even wood nest eggs. Some people have even had success with rocks or golf balls. 

If your hens are currently laying their eggs somewhere you don't want them to, try to block that area off or make it less attractive somehow.