Then you should search the forum for "BGA". I'm sure you will be able to find advice for yourself. It has been gone over a lot in various forums.
Here's a discussion I found quickly using the search:
There are others.
While I advise you to search, I'll also post some info here anyway since I know you want to start working on the problem immediately and may not be familiar with how to use the search yet.
Add KNO3, your nitrates are probably low.
If you choose to blackout, here's a rundown that I ripped from a post Tom made on another forum. I added the stuff in  to clarify things since the post has been taken out of context.
See Planted tank net, APC etc.
I've repeated it there a dozen or more times.
If you catch it [BGA] early, you never have to do it [blackout], simply suck it up and irritate it, water change, then add KNO3.
If you got it bad, then the blackout.
50% [water change] before and get all of it off you can, clean filters, vac up old substrates if gooey etc, then blackout(cover the tank!!!! so that no light gets in at all) for 3 days and add the KNO3(1/4 teaspoon per 20 gal).
Turn CO2 off, aerate or increase circulation.
Remove cover [after the full 3 days of blackout], add light back, do 50% water change, Add KNO3 back and then there after, keep filters clean regularly. Remove extra circulation, add CO2 back, if you have plants at or near the surface, trim them back prior to any of this.
That is pretty much it.
Most failures are from: not doing the full 3 day, not covering the tank so that no light at all gets in, not adding the KNO3, during or after.
Some use the Antibiotics in conjunction or with the KNO3 so that it does not come back.
Either method works but the KNO3 and dirt issues where never addressed with the pills, the blackout helped to find the long term cure, thus no need for drugs thereafter.
Some clients don't want to do the blackout, so I had no choice, but I prefer the blackout personally.
If there is BGA after you remove the cover, you did something wrong.