Thank you. I understand you have good understanding of MongoDB and Teiid. If you have not worked with Teiid, I suggest going through "Quick Start Example" first and know how Teiid works. Look through the Reference Guide. For developing a Teiid translator, you need to read relevant sections in "Developer's Guide".
Some instructions on starting a translator are here http://community.jboss.org/message/555888#555888
Take a look at the sample projects mentioned for sample code or examples, and ask questions.
For contributing code to Teiid, you would need read and agree for JBoss contributor license agreement at https://cla.jboss.org/login.seam?cid=1662