if my building has wifi does it mean am sharing the same ip address with all?

If you use the building's internet connection, then yes, you will be sharing the same IP address. But you can have your own internet connection if you want so that you can have your own IP address for your online work.

Yes an IP address is the "Internet Provider Address", it is nothing to do with an individual computer or laptop, but the internet router basically. Also, yes you can have your own internet connection, this may be interferred with by the other wifi, but as soon as you tell your computer "not to connect" to the main buildings internet, and only to connect to yours, the only time it would connect to the buildings wifi would be when your own was not working. You wouldn't lose your own websites, i.e. if you have made 50$ on here, on the main wifi, you would still have these earnings on your account no matter where you are using your computer! :)

No, it is not completely true. But the thing is that others might take advantage of your wifi connection...if they come under its range!

Yes. You're sharing ip addresses. Although you can always use a proxy ip. :)