Akhtar,
Don't panic. B) Snails have their benefits, and many aquarists keep them intentionally. It is very, very common for snails to find their way into aquariums hidden on plants. Most snails are algae-eating machines, and they'll help keep your tank clean by eating all the edible detritus in your substrate. Of course, as everything that has benefits does, snails have some drawback as well. The main thing people dislike about them is that certain species can reproduce at astounding rates, and can leave you with a snail overload. This won't happen overnight though, it takes some time. Also, some species of snails can wreak serious havoc on certain plants.
I highly recommend doing some reading at
http
/www.applesnail.net. It's agreat site, and will give you enough knowledge to figure out where to go from here.
There are several ways of getting rid of snails. There are a number of snail-eating fish that will eliminate a snail population in no time flat. There are also chemicals (blech!)

available that will get rid of them. I hear they do work, but they personally wouldn't be my first choice.
pendragon!