Where to write codes and make apps on computer? Do you have to buy a software?

Answer 1

For java you can use eclipse or NetBeans

Answer 2

You need a IDE (simpy: tool for programming/development environment). There are many different ones for every language. Theoretically you could just use a normal text editor like notepad but it is not efficient. For Java you could for example use: Netbeans, IntelliJ, Eclipse, Notepad++, BlueJ...