The first step in opening a file which is of an unknown file type is to figure out what kind of file it is and what kind of program is best used to open it. Thousands and thousands of file types have been created over the years - often people and companies will create new file formats for the programs that they are writing. The most common file formats are those which are accessible through many programs and which have been specially developed to be good at representing a certain kind of information. To find out about file formats and their details you can go to file format library sites on the internet.
A simple internet search for the file extension of an unknown file type will usually bring you results and explanations. To get the file type all you need to do is right click on the unknown file and open the "Properties" dialog box. From there you can find the three or four letter code on the end of the file which corresponds to the type of unknown file that it is. Search for that file extension and consider adding words such as "file" or "player" to get better info back from your search. If you find the file type on the internet you are likely to immediately find details on the kinds of players that can read it as well.
If your mystery file format is some kind of graphics file, audio file or video file chances are that you will be able to read it with a common program that can be downloaded. Downloading a free or shareware program from the internet will let you open the unknown file and give you a program that you can associate with the file so that it will appear as a known file in Windows Explorer and not just a blank page.
If you are having trouble finding the info on the file type online, you may want to think about where you got the file from. Did the file come with a program? Can you ask the person who sent you the file what to do with it? Sometimes even pulling up the Properties window will not give you the full story on the extension and file type, or a file may be identified with the wrong file extension in its title. In cases such as these, you may have to get help from someone who knows the file, or get another copy.
With lots of free and shareware software on the internet you should be able to easily open any kind of common file format. If a file format is proprietary you may need a particular program in order to run it. Often programs will let you associate the files that they can open with them automatically as part of the set up and the first time they are run, while other times you must use the "open with" dialog by right clicking on the file you want to associate. Use these tips well to access your mysterious unknown file types!