First you have to repent (at-tawba) from sin.
Second you should expiation vow with:
- Feed or clothe ten poor;
- If you can't you should fast for three days.
For second question, if you don't make an oath you should just repent (at-tawba).
And if you end up back again, repent (at-tawba).
For more information, see the YouTube videos What is the kaffara for a broken promise made to Allah by Sheikh Dr. Muhammad Salah, Learn Islam - Q&A, and Repentance(Tawbah) By Mufti Menk.
Note: Vowing is not good in Islam just say "in·shal·lah" I won't do that, meaning if Allah wills it.