How to perform load testing for website using JMeter

Question

I want to create a script in JMeter that simulates e.g. 100 users accessing our site over a period of 3-4 minutes.

We need to test whether our application can allow 25.0

Answer 1

1. I would advise to use better some custom samplers from jmeter-plugins instead:

• Ultimate Thread Group: more flexible implementation of Thread Group allows to setup your load-testing scenario exactly how you want;
• Response Times Over Time / Response Times vs Threads / Active Threads Over Time listeners: more demonstrative graphs as JMeter's default ones.

2. In addition to the @Qwe comment above: test not the single HTTP request but user's transactions: each test-thread should simulate a kind of real-user browsing activity: login (if any), navigating through the site, search, accessing/editing personal settings, logout, etc.
To emulate real-user add also "think-time" timer between transaction steps.

3. Control responses, at least the following:

• response code of each user's request (@ Response Assertion);
• duration - to verify non-functional requirements for critical transactions (@ Duration Assertion).

4. Look into corresponding JMeter documentation:

• My boss wants me to load test our web app!
• JMeter Wiki

5. Some step-by-step guidelines about Jmeter setup and usage for load-testing you can find here:

• Load Testing your Applications with Apache JMeter
• Using JMeter
• Effective load testing with Apache JMeter
• Load Testing with JMeter: part 1, part 2, part 3
• A Simple Load Test with JMeter
• Load Testing with Apache JMeter
  Performance testing with JMeter
• Your First JMeter Test
• Some thoughts on stress testing web applications with JMeter: part 1, part 2.

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UPD.
Please find, use and feel free to extend this Awesome JMeter collection continued on github.