There are three basic things you need to remember when using IRC commands:
All commands begin with a forward slash '/' everything else is considered text and displayed in the channel you are in.
Nearly all channel names begin with the '#' sign. The exception to this rule are channels that start with '&' or '+' but you will rarely run across those in your IRC life, as they are generally not used. & channels are where you cannot see who is in the channel. + channels are channels local to the server.
IRC is not case sensitive, you may type caps, lower case, or a mixture of both. Note however that using all caps when you are chatting is considered as shouting and often rude
Here is a list of BASIC IRC commands that you should find helpful:
/join #channelname
Use this to join a channel, example: /join #Beginner
/part #channelname
Use this to leave a channel, example: /part #Beginner
/leave #channelname
Same as above, example: /leave #Beginner
/quit (message)
Use this to quit IRC. A quit message may be added to the command, but is optional. example: /quit Have to go to school but will be back later!
/whois nick
Used to get info on a nick. example: /whois space
/whowas nick
Used to get info on a nick that has just left IRC or a person who has just changed their nick, example: /whowas nicky
/who #channelname
Shows nicks, addresses, and optional user info if available for users on a channel, example: /who #Beginner
/names #channel
Shows you who are on a channel, example: /names #Beginner
/msg nick message
Sends a private message to a user. example: /msg Nicky can you help me?
/query nick
Opens a new window for private dialog with a nick. example: /query Nicky
/nick newnick
If I wanted to change my current nick, Nicky, to Cat, I would use this command. example: /nick Cat
/me action
This is how you get those actions which appears as purple. If you wanted something like: *Vicky says hello to Angela you would type it like this: /me says hello to Angela
/away away_message
Use this to tell others you are not currently at your computer, example: /away just rinning down to the shop to get some chocolate. To indicate that you are back, type /away .
/list
This will give you a list of open channels on the network.
/invite nick #channelname
Use this to invite a nick who is elsewhere on IRC to join you on a channel. example: /invite rogue #Beginner
/ignore nick/address
Use this to prevent someone from talking to you. The person's nick or address may be used, example: /ignore lamer or /ignore *!*sexy@badusers.com
/kick #channelname nick
Used to kick a user from the channel. Note that you must be a channel operator to use this command. example: /kick #Beginner lamer
/mode nick +i
This is the invisible mode, but you are not literally invisible on IRC as users on the same channel as you still see you there. You are invisible to a user who does a /who or/names if they are not in the same channel as yourself. Note also that /whois yournick will show your user information and the channels you're on.
For help on the Nickname you've chosen, type /msg nickserv help
Here you will learn how to register your nickname, set security on your nickname, change your nickname..
For yet further help on mIRC commands, please check the mIRC help facility. Don't be scared to play around with the settings of your client. Customise it to suit your needs.