Although this is one of the most important aspects of your day paintballing, it is also one of the most simple.

ALWAYS wear your goggles when instructed to, if you aren't sure, then either ask a marshal, or just wear them. Never, ever be afraid to shout at someone who has taken their goggles off. They deserve to be called every name under the sun.

During the safety briefing you will have the marker explained to you. Take note of any mention of a safety catch, and use it. At most sites, the markers are handed in at around waist level, if you accidentally pull the trigger then you will not be thanked by the (now high-pitch voiced) marshal.

Take note of any surrender rules in force. It's not particularly pleasant to be hit from about three feet.

If your marker starts leaking gas, put it on the floor and shout for a marshal. It usually isn't dangerous, but dealing with high pressure gasses can be harmful to your health if you don't know what you're doing.

Apart from that, it's all just common sense. Have fun, and stay safe.