What program can I code HTML in?

Any text editor is usually capable of writing HTML.

Notepad and Wordpad (Windows and Mac programs, respectively) are the most common text editing programs.

However, there are text editors that have been made for use by web designers - by highlighting tags, colouring sections, identifying the ends and starts of brackets and parenthesis, and more. These programs will need to be searched for, to find what the true web designer wants. Many of the text editors may need to be paid for, but others are free to use.