The numbers may vary depending on your belief. 11 and 3 are known asAngels numbers and 12 is known as acompletion. Numbers have many meanings behind them, and can be a bit tricky to identify for another person. There's a lot of information on the webs that should help you. Just type up anumerology meaning, for more information.
Again it may vary on your belief. Either your angels or spirit guides are trying to get your attention. It could also mean that there's a very important lesson behind the number. So I recommend a google search, for whatever number you are seeing on the web, and see what answer is the most relevant in your life at the moment.
If you want to learn about numerology, Linda Goodman was one of the premier authors on the topic. Her books can take some time to digest though as her writing style tends to make things seem more complicated than they are.
She espouses hebrew chaldean numerology methods and I find them to be pretty useful. I think 'star signs' would be a good place to start. It covers a wide range of topics and kinda shows how much overlap there is between astrology, numerology, and writing/language.
Someone made a simple website that I found a while back that might help as well. It is a basic numerology calculator that spits out direct exerpts from Linda Goodman's book about each number. (you'll need to copy-paste the url as links are only allowed through certain people on the site.)