This article answers your first question. Read the summary of a few of the tablets near the end.
http://www.socialstudiesforkids.com/articles/worldhistory/twelvetables.htm
Check this article for the answer to your second question.
http://oncampus.richmond.edu/academics/education/projects/webunits/greecerome/Romegovt1.html
What did the 12 tables do fro the rights of plebians?
Who could vote in rome?
2 answers
Here are articles on the Twelve Tables of Roman law:
http://www.google.com/search?hl=en&as_q=twelve+tables+rights+plebians&as_epq=&as_oq=&as_eq=&num=10&lr=&as_filetype=&ft=i&as_sitesearch=&as_qdr=all&as_rights=&as_occt=any&cr=&as_nlo=&as_nhi=&safe=images
Articles on who could vote in Rome:
http://www.google.com/search?hl=en&lr=&as_qdr=all&q=who+could+vote+in+rome
http://www.google.com/search?hl=en&as_q=twelve+tables+rights+plebians&as_epq=&as_oq=&as_eq=&num=10&lr=&as_filetype=&ft=i&as_sitesearch=&as_qdr=all&as_rights=&as_occt=any&cr=&as_nlo=&as_nhi=&safe=images
Articles on who could vote in Rome:
http://www.google.com/search?hl=en&lr=&as_qdr=all&q=who+could+vote+in+rome