The things you need to know depends on what designation you’re hired for at an IT company. However, as far software part is concerned you must start with basic programming languages such a C, C++ and Java.
Programming is just one part of the equation in a software company. You should be thinking about learning things such as software development life cycle, project planning and scheduling, task tracking as well as how to work in teams if you want to be a valuable contributor to the company.
Basic knowledge of application development for iOS, Android or Web is also expected from the candidate. You must also be able to demonstrate strong organization skills, attention to detail, and strong verbal and presentation skills.
Ultimately, whoever they hire as a developer, what most software companies want in him or her is a serious passion for coding, with the ability to intuitively pick up and use any new language/protocol/framework.