The program you use for editing text is, unsurprisingly, a text editor.
A text editor is different from a word processor. A word processor is concerned with how the text looks (fonts, sizes, layout). All a text editor is really doing is editing the text: that is, shuffling the characters around.
Text editors typically use a monospace font: that means every printable character occupies the same width. The text file is therefore displayed on a grid.
This is useful because it means you can identify the position of a character in the file by its line number, and its column number. This information is often provided in diagnostic messages (or error messages) when you are programming.
Using line:column to identify a location within a text file is an implicit feature of the text file format.
There are many different text editors. Find one that you like (maybe as part of an IDE if you’re using it to program), that runs on your machine, and learn to use it well.
The basic text editor on Windows is Notepad. On MacOS it is TextEdit. Both these are quite limited, and you should probably investigate getting a more powerful one.
Examples include Notepad++, Atom, BBEdit, Kate, Visual Studio Code.
A non-GUI text editor
It’s handy to know how to use a text-only editor too — that is, one that does not use a Graphical User Interface (GUI). It allows you to edit files even when you are connected to a remote server via a simple terminal.
The two most common are emacs and vi (or vim). Each has its pros and cons but the main benefit of knowing how to use one of them is that it will almost certainly be installed on the remote server you’ve logged onto.
Text-only editors like these take a little more effort to use efficiently, because you must learn the basic keys needed. Both are powerful tools for editing files, but you need to practice to get familiar with how they work.