IRC has users from all over the world, from all different cultures, all languages, all ages, and all economic backgrounds. It is important to follow some basic rules of etiquette so everyone can participate and engage effectively.
Most established channels publish a set of rules. Upon first entering a channel, look at the channel topic and see if they mention a web site, or possibly a command that will display the channel rules to you (!rules is quite common). It is important to read and follow the channel rules. If you disagree with them, then that channel is probably not for you. Breaking the rules will normally get you a warning from an Op, followed by a kick from the channel, then a ban if you continue to break the rules. Some infractions may deliver an immediate ban, depending on the severity of the infraction or the importance of the rule. If you don't see anything about rules, ask an Op (identified in channel by a @ in front of the nick name), or ask in the channel if there is a written set of rules.
It's always a good idea to “lurk” for a bit and see how the channel operates. You'll be able to watch others break rules and get warning, kicks or bans, and you'll start to understand the personality of the channel. You can also introduce yourself – keep it short and to the point, and try to include your connection with the channel topic.
English is generally the preferred language on the major IRC networks. Some networks are for specific regions or countries and will have their own preferred language across the network. Some channels have English usage as a rule in the channel, while any language can be used in personal chat/message windows. Channels can also be language specific – there are quite a few channels where French is the primary language, and others where they allow a mix of English and Romanian, as an example. If you don't speak the requested language, the channel is likely not for you. Most channels won't mind if you give a simple greeting in English; they don't want entire conversations going on in a non-required language.
Here are some things that will almost always get you a warning, kicked from the channel, or outright banned – I think these are all self-explanatory and should be fairly obvious to anyone that cares about getting along on IRC:
Flooding – repeating the same text over and over.
Disrespecting other channel users, especially channel Ops.
Using foul language if the channel doesn't allow it (e.g. adult channels).
Intentionally disrupting the channel.
Posting links to images, videos or websites in channels that don't specifically allow that.
Racist or bigoted language and behaviors.
Incessantly private messaging other channel users who don't want your attention.
Sending picture links to channel users who don't want them.
Posting spam or advertising in the channel.
Adult-oriented nicknames in non-adult channels, or underage nicknames.
Incessant arguing – some channels have an outright ban on religion and politics for this specific reason.