What is the risk of doing a rebuild to my indexes? What is the risk of doing a rebuild to my indexes in SQL Server 2012?
I have to do a REBUILD, but I don't know what are the risks.

The server is running and a lot of information is going in and out.
Without knowing details about exactly what type of queries are accessing that data and the indexes you are dealing with a precise answer is not possible. 

However, in general, an index rebuild drops and then re-adds that index and locking needs to occur for that to happen. If you have queries that are attempting to access this table while the rebuild is going on, they will likely need to wait for it to finish before they can complete.

I found a very good rundown of the locking in a blog post by Kendra Little; and under the "Online" section of the ALTER INDEX MSDN page you can see Microsoft's description of the timing of these locks.

If you are using Enterprise Edition, you can run the index rebuild (or even create) in online mode (with hint Online = on) and it will not cause blocking.