What atones for wilful sin?

According to the Torah is there any procedure which we have to go through for wilful sin to be forgiven like a sacrifice or so forth?

Shalom, Thank you for your question. There certainly is a process to go through in order to be forgiven for all sins – both willful and careless sins. It would not be an exaggeration to say that this process is at the very foundation of the Torah, and it is possibly the most important message at the very root of Judaism. In Hebrew it is known by the name "Tshuvah" – which is usually translated as "repentance", but would be better called "return". Returning to G-d, from whom one distanced themselves through sin, and returning to oneself, from whom we strayed. I would suggest first that you read the Rambam's code concerning the laws of Tshuvah, ten chapters (which can be found in English at the following website - http://www.chabad.org/library/article_cdo/aid/911887/jewish/Teshuvah.htm ) After this, may I suggest you try to find a copy of Rav Kook's famous work "Orot HaTshuva" (The Light of Repentance), which has also been translated into English several times. This classic text sheds a new and wondrous light on the whole concept of Tshuva, and explains it in a spiritual, psychological and practical way. You can find classes on this book at the Machon Meir website – http://www.english.machonmeir.net/torah-lessons-archive?view=jmultimedia&catid=297 I hope this will be a good start to your learning about this great subject – Blessings.