With iMessage you will only be able to send messages to others who have an iPad, iPhone, or iPod. They also need to be on IOS5 and have this feature activated. Are you on IOS5? Have a look in Settings - General - About and see what version you're on. If you are have you activated it? Look in Settings - Messages and let us know.
The IPad Forum is a community of members from all walks of life and from all over the world coming together to share our experiences and to help each other learn about this wonderful piece of technology the Ipad. There is a wealth of information already here in the many threads we have so doing some searches will answer most of the questions you have. If then you cannot find the answer you are seeking then by all means post your question. There are many friendly and informed members here only too willing to help you.