You are free to choose any nickname you like, up to 9 characters long. It is also very common to find people that use the same nickname and you may be asked or choose to switch nicknames to avoid confusion. It's probably best to take a look around and see what you want to do first. Follwing the channel name you will see the number of people on it as well as its topic. A channel is a 'place' on IRC where group conversations occur.
People can join the same channel and see each other.
Depending on its topic and time of the day a channel can be VERY crowded. Channels can alsobe quite chaotic, or calm. Channels can be open to everyone but also closed and private and only open to friends. Channels on IRC are dynamic in the sense that anyone can create a new channel, and a channel disappears when the last person on it leaves. Now that I've decided on a nice channel.