What must a former Christian do to become a Bnei Noach and have a place in the world to come if he/she violated any of the mitzvots? Are we allowed to attend a local synagogue to learn what he/she is allowed to do from a Jewish Rabbi?

Shalom, Thank you for your question. According to Judaism, a non Jew also has a place in the world to come. We believe that not all humanity should serve G-d as Jews – but that most of humanity should follow the Noachide laws. If a person has broken some of these laws, then they should repent of their sin – through verbally expressing their regret to G-d (in prayer, alone), and if there was also damage to other people (such as theft etc), then repairing the damage they did (paying back the theft etc). In general a Rabbi would be happy to meet you and give you advice on how to serve Hashem. Feel free to seek out your local Orthodox Rabbi, and give him a call to arrange a meeting, or phone conversation. Blessings.