Ask your web hosting provider for raw log files. This will contain everything you need to know about your website: error messages, file sizes, bandwidth, visitor details, etc.

If that is not possible for some reason, then you could add any one of the countless different stat counters on the download page itself. But what I would recommend is that you remove the ability for people to download, and instead use an online file sharing or file storage website that pays you pennies for each download. That way you are getting paid for each download, even if it is only a little bit. It takes the load off your own servers, and allows many more people to download at the same time. Do an Internet search for "get paid to share files" or something similar and you will find ziddu.com and many others.

you can use a script or I can do it for you if you want

You can just take scripts .. google for " Free Download counter script " or " free link click stats " or something similar