Not enough information.
It may be a common file type, shared by many applications, and in that case Googling the file extension is sometimes enough. Unfortunately, ".db" is an extremely generic extension (compare, for example, ".txt" versus ".wp5"). In this particular case, you inspect the file with a hex viewer and try to find documentation based on the very first couple of bytes.
If the data format is known, chances are someone already wrote a converter for it. But before you can try one of these, you must know beyond a shadow of a doubt what kind of file it is; "oh right, then it's a .db file" is not enough.
If it's a private format (or it's so old/new/quaint/rare that you can't find anything useful on the 'net), you will have to write something yourself to (1) read out useful structures, and (2) write out its data in a format useful to you. Recognizing what parts are useful and what's not is something only you can do, as (apparently) you have an inkling of what it should contain.