Summarize:

HTML is the basic language that all web pages are made with. Knowing HTML basics can help you identify weaknesses in websites, as well as code your own basic sites. See How to Learn HTML to get started. JavaScript is one of the main ways that content is displayed on websites. Learning how JavaScript works can help you identify exploitable parts of websites and inject your own scripts. See How to Use JavaScript Injections for a look at how injections work. SQL is the main database language for websites and online services. Hackers spend a lot of time attempting to access databases, so knowing how SQL works is essential for most hacking. See How to Create a SQL Server Database for information on setting up an SQL server to test. PHP is a server-side language, and is often used to handle secure logins. This makes knowing PHP essential for hacking secure sites. See How to Learn PHP and MySQL for details on getting started with PHP. This is one of the most essential languages for hackers. Python is a high-level language that allows you to quickly write programs and scripts. It is essential for writing your own exploits and hacks. See How to Start Programming in Python for tips on getting started.
Learn HTML. Learn JavaScript. Learn SQL. Learn PHP. Learn Python.