I am new in the website scalability realm. Can you suggest to me some the techniques for making a website scalable to a large number of users?
A number of people have mentioned tools for identifying bottlenecks, and that is of course necessary. You can't spend productive time speeding something up without knowing where it's slow. But the other thing you need to know is where your target scalability lies. Is it value for money to spend a couple of months making your site scale to the same number of users as Twitter if it's going to be used by three people in HR? Do you have a known rate of transactions, or response latency, or number of users, in the requirements of the product? If so, target those numbers with your optimisation strategy. If not, find those out before chasing the performance rat down the hole.