Several popular programs are mIRC , IRCle , pIRCh , and macIRC . Once you install a client, you need to get onto a server that will connect you to a network. There are several IRC networks out there. Dalnet is the most popular one currently, though EfNet, UnderNet, and AustNet are in use by IRCers too. For the most part, if you are on one network, you cannot be seen on the other networks.
Servers are the pathways onto the various networks.
If you are on one server for DalNet, you should be able to communicate with people on other servers for DalNet. The current reliable server is hebron.in.us.dal.net. Other DalNet servers include viper.mo.us.dal.net and liberty.nj.us.dal.net. Each IRC client has its own way to join channels. This FAQ will deal with the most popular channels.