Here is a helpful website:
http://www.irchelp.org/
Plus, if you want to recieve a file you will have to configure Mirc.
Before you download, you need to fiddle with mirc so it will download your file.
go to DCC-->Folders
there is a file ignore list there, you can add the extensions of the files you want
make sure you uncheck the box below it.
Many channels are different, make sure you read the rules before you do anything. If you aren't sure, ask. People will help you.
If you are looking for movies, i know of a place on DALnet, but you will have to
register first. Not to scare you, the registration is required for this channel.