You need to be more specific but I'll try. I assume Windex isn't working for you, this is common... My car for example had 100K on it when I got it and there was a film inside that did not want to come off. There are several things to try, one is buy a paste cleaner for glass, it's like paste cleaner/wax for a cars paint but just for glass. It dries to a powdery finish which is easily wiped off and usually takes whatever with it. If that doesn't work try brake cleaner, its an excellent cleaner for many things, but it can eat certain plastics so be careful. It will dissolve most anything on glass so use a paper towel with some brake cleaner on it, wipe a section and toss the towel. The reason is the towel won't absorb much gunk and then you're just smearing it around. Another great tool is a single edge razor blade, a new blade does an excellent job of scraping the surface, so well that it can even get a thin film off.
If you can't get whatever off with brake cleaner or a razor, then I'm at a loss.
The good thing about cleaning glass with these things and getting it honestly clean, is that basic cleaners like Windex will usually work fine after that. One trick I learned with Windex is to use news paper to wipe with. Not sure why but it leaves a better streak free and lint free finish.
One more thing; one the glass is truly clean, use automotive wax on it just like you would on paint. This will be invisible and will protect the glass from more crap sticking to it for easier cleaning next time. If it's waxed just use mild soap and water because Windex etc will strip the wax off, but if it's really dirty then that's a good thing because with the wax come the dirt.
If the glass is exterior like on a car, it'll usually have little chunks of crud that you can feel with your fingernails that cleaners won't remove. You can either use a razor blade, or better yet use a clay bar that's sold for auto paint. Great stuff, just follow the instructions and wipe the glass with it and it magically removes all those micro chunks. Those chunks are probably all over your cars pain as well, just wash the car and then use it. It's not a big project, it works vary fast.
Hope this helps...