Final answer:
To open a file, double-click it if it has an associated program or use 'Open with' to select an appropriate program, using software like Notepad for text files, or Word and Excel for document and spreadsheet files.
Step-by-step explanation:
To open a file called "filename", you would typically use a software program that is appropriate for the file type. On a Windows computer, you can double-click the file if the file extension is associated with a specific program. On a Mac, you can also double-click the file, or you can use the 'open' command in the Terminal. If the file does not open automatically, you can right-click on the file and select 'Open with' to choose a program that can handle that file type. For text files, a simple text editor like Notepad on Windows or TextEdit on Mac can be used. For other file types, such as .docx or .xlsx, programs like Microsoft Word or Excel are more suitable. It's important to note that trying to open a file with a program that is not meant to read that file type may result in an error or unreadable content.