API Testing for Manual Testers: 7 Easy Steps to Master API Testing

API Testing for Manual Testers: Complete Beginner Roadmap

Do not try to learn everything at once.

Contents

Follow the roadmap step by step.

Step 1: Understand Client-Server Architecture

Before learning tools, understand the basic communication flow.

Client
   ↓
Request
   ↓
API / Server
   ↓
Business Logic
   ↓
Database
   ↓
Response
   ↓
Client

Understand these basic terms:

Once this flow becomes clear, API testing becomes much easier.

Step 2: Learn HTTP Basics

HTTP is fundamental to API testing.

Start with the commonly used HTTP methods.

GET

Used to retrieve information.

Example:

GET /users

You might expect a list of users in the response.

POST

Used to create new data.

Example:

POST /users

Request body:

{
  "name": "Mrunal",
  "email": "mrunal@example.com"
}

PUT

Generally used to update an existing resource.

PUT /users/101

PATCH

Used to partially update an existing resource.

PATCH /users/101

For example, you may update only the user’s email.

DELETE

Used to remove a resource.

DELETE /users/101

For a detailed explanation of HTTP request methods, refer to MDN's HTTP request methods guide .

Step 3: Understand API Request Components

An API request can contain several important components.

1. URL / Endpoint

Example:

https://example.com/api/users

The endpoint identifies the API resource you want to access.

2. HTTP Method

For example:

GET
POST
PUT
PATCH
DELETE

3. Headers

Headers provide additional information about the request.

Example:

Content-Type: application/json
Authorization: Bearer token

4. Query Parameters

Query parameters are often used to filter or modify a request.

Example:

/users?page=2&limit=10

Here:

page = 2
limit = 10

5. Path Parameters

Path parameters identify a specific resource.

Example:

/users/101

Here:

101

may represent the user ID.

6. Request Body

A request body contains data sent to the server.

Example:

{
  "name": "Mrunal",
  "city": "Pune"
}

Step 4: Learn HTTP Status Codes

Understanding status codes is extremely important for API testers.

You don’t need to memorize every status code initially.

Start with the commonly used ones.

2xx — Successful Requests

200 OK

The request was successfully processed.

201 Created

A new resource was successfully created.

204 No Content

The request succeeded but the server has no response body to return.

4xx — Client Errors

400 Bad Request

The request is invalid.

401 Unauthorized

Authentication is missing or invalid.

403 Forbidden

The user is authenticated but does not have permission.

404 Not Found

The requested resource does not exist.

5xx — Server Errors

500 Internal Server Error

Something went wrong on the server.

503 Service Unavailable

The service is temporarily unavailable.

Step 5: Learn JSON

JSON is extremely common in modern APIs.

Example:

{
  "id": 101,
  "name": "Mrunal",
  "role": "QA Engineer",
  "experience": 4
}

As a beginner, focus on understanding:

Example array:

{
  "users": [
    {
      "id": 1,
      "name": "Amit"
    },
    {
      "id": 2,
      "name": "Priya"
    }
  ]
}

You do not need advanced programming knowledge to start working with JSON.

Step 6: Learn Postman

Once you understand HTTP and JSON, start using Postman.

Postman is one of the most popular tools for API development and testing.

With Postman, you can:

Your first goal should be simple:

Send a request and understand the response.

Do not jump into automation immediately.

You can learn more about sending API requests and testing APIs in the official Postman documentation.

Step 7: Practice GET Requests

Start with GET APIs.

For example:

GET /users

Check:

Request

Response

Step 8: Practice POST Requests

Next, learn how to create data.

Example:

POST /users

Request:

{
  "name": "Mrunal",
  "email": "mrunal@example.com",
  "age": 30
}

Verify:

Step 9: Learn Positive and Negative API Testing

This is where your manual testing experience becomes extremely useful.

Positive Testing

Send valid data.

Example:

{
  "email": "user@example.com",
  "password": "Valid@123"
}

Expected result:

Login successful

Negative Testing

Send invalid data.

For example:

Example:

{
  "email": "",
  "password": "123"
}

The API should return an appropriate error response.

Step 10: Learn Authentication

Authentication is an important part of API testing.

Common authentication approaches include:

For beginners, start with:

API Key → Basic Auth → Bearer Token → JWT → OAuth

Understand how authentication affects API requests and responses.

For example:

Authorization: Bearer eyJhbGciOi...

You should test scenarios such as:

Step 11: Learn API Variables

Variables make your Postman collections reusable.

Instead of writing:

https://qa.example.com/api/users

everywhere, you can define:

{{baseUrl}}

Then use:

{{baseUrl}}/users

You can also store:

{{token}}
{{userId}}
{{email}}

This becomes especially useful when working with different environments such as:

Development
QA
Staging
Production

Step 12: Learn API Chaining

API chaining is an important real-world concept.

Suppose you have:

API 1 — Login

POST /login

Response:

{
  "token": "abc123"
}

API 2 — Get Profile

GET /profile

API 2 requires the token generated by API 1.

The flow becomes:

Login API
    ↓
Extract Token
    ↓
Store Token
    ↓
Send Token
    ↓
Profile API

This is called API chaining.

Step 13: Learn Response Validation

Do not simply check whether the API returns a response.

Validate the response properly.

For example:

Status Code

Expected: 200
Actual: 200

Response Field

Expected name: Mrunal
Actual name: Mrunal

Data Type

If age should be a number:

"age": 30

it should not unexpectedly become:

"age": "thirty"

Required Fields

Verify that important fields are present.

Step 14: Learn Boundary Testing for APIs

Your manual testing skills are very useful here.

Suppose the requirement says:

Username must contain between 5 and 20 characters.

You should test:

4 characters  → Negative
5 characters  → Positive
6 characters  → Positive
19 characters → Positive
20 characters → Positive
21 characters → Negative

You can apply the same boundary-value technique to:

Step 15: Learn Data-Driven API Testing

Instead of manually testing one data set, you can test multiple inputs.

For example:

UsernameExpected Result
user123Success
testuserSuccess
“”Error
abcError
verylongusername…Error

This approach becomes extremely useful when you start automating APIs.

Step 16: Learn API Automation

Once you are comfortable with manual API testing, move toward automation.

You can explore tools and frameworks such as:

If you are already learning Playwright with TypeScript, API testing with Playwright can be a natural next step.

For example, Playwright provides an API request context that can be used to send requests without opening a browser.

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