Hello,
Follow these steps,
1. Start Notepad (or a better .txt editor, like Editpad).
2. Opening .DAT file with Notepad
3. Open the .DAT-file. If this succeeds, try to spot clues in the file to
figure out what kind of data you are dealing with. Proceed accordingly.
If you couldn't open the .DAT file with a text-editor (Notepad), try as many
other programs as you like. Suggested programs to open the .DAT file: MS Word
or Excel (because some .DAT file's are comma-separated-value (.CSV) files
..DAT files could also be MS Outlook-files. You could also try an IDE
(Itegrated Development Environment, a programmers tool).