In general, you open document and template files the same way you do in other programs. When you open an InDesign template, it opens as a new, untitled document by default. In Windows, document files use the extension .indd, template files use the extension .indt, library files use the extension .indl, and book files use the extension .indb.
You can also use the File > Open command to open files from InDesign 1.x and later (including Asian-language versions), InDesign Interchange (.inx) files, Adobe PageMaker 6.0 and later, QuarkXPress 3.3 and 4.1, and QuarkXPress Passport 4.1. In addition, other software vendors may make plug‑in software that lets you open other file formats.