In this article i am going to quickly show you how you can build a simple rails api. This will give you an idea of what building API entails.
I am going to make this as simple as possible so that you understand the basics.
The api end point that we will be building will get us a random greetings.
Lets dive in and get our hands dirty.
First, you will need to create a Rails API app by running the following command:

rails new my_api --api --database=postgresql

We will need to create the database by running

rails db:create

Next, you will need to create a model for your greetings table by running the following command:

rails g model Greeting content:string

Run the following command to create the table in your database:

rails db:migrate

Next, you will need to seed your database with some greeting data. You can do this by adding the following lines of code to the db/seeds.rb file:

Greeting.create(content: "Hello")
Greeting.create(content: "Hi")
Greeting.create(content: "Good morning")
Greeting.create(content: "Good afternoon")
Greeting.create(content: "Good evening")

Run the following command to seed your database:

rails db:seed

Now you can create your API endpoint. In your config/routes.rb file, add the following line of code:

get '/random_greeting', to: 'greetings#random'

Create a GreetingsController by running the following command:

rails g controller Greetings

In your app/controllers/greetings_controller.rb file, add the following action:

def random
  greeting = Greeting.order("RANDOM()").first
  render json: { greeting: greeting.content }
end

Now when you make a GET request to the '/random_greeting' endpoint, it will return a JSON response with a random greeting from your table.
I hope this helps! Let me know if you have any questions.