What is sin? What does Yeshua (Jesus) expect of those who belong to Him?

Yahweh is the God of the bible: the God of Abraham, Isaac, and Israel (Jacob)
Yeshua is the Messiah.  His name is translated as Jesus in english.  I prefer to use Yeshua to reflect His Jewishness and the Jewishness of following Him although there is nothing wrong with using Jesus as His name. 

Turning away from sin means to turn away from violating Torah - 'the law' - meaning Yahweh expects us to obey His rules He gave us in the Old Testament

1 John 3
4  Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.

Matthew 7:21-23 Yeshua says He will REJECT those who claim to be His if they do not obey Yahweh

Matthew 7
21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.

Verse 21:  Not everyone who claims to belong to Messiah Yeshua is accepted by Him as one of His followers.  In this verse, Yeshua says only those who obey Yahweh, the Father, will enter the kingdom of heaven.  Obedience to the Father is obeying Torah, the instructions found in the first five books of the bible:  Genesis, Exodus, Leviticus, Numbers, Deuteronomy

Matthew 7
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?

Verse 22: Here many people who claim to be Yeshua's followers and are doing things "for Him" are bringing up their works - in context they are at the the judgment and are using this in their defense because they know they are going to be sent to hell.  They are surprised that they were not saved even though they were "in ministry" and "doing things" for Yeshua.  This verse is about most people who claim to be Christians:  those people are rebels and say "we do not have to obey God because Jesus died so we don't have to" (words to that affect)

Matthew 7
23 And then will I profess unto them, I never knew you: depart from me, ye that work 
iniquity G458.

     Strong's number G458 :  iniquity (transliterated as anomia) is 
     anomia
          1) the condition of without law
          a) because ignorant of it
          b) because of violating it
     2) contempt and violation of law, iniquity, wickedness

Verse 23: Yeshua will reject those who do not follow Yahweh's instructions (the Torah / the Law)  Yeshua is very clear in verse 22 - where people were doing works "for Him" and verse 23 - where He rejects those who do not obey that there is a vast difference in attempting to do works without obedience for salvation (which is no salvation) and obedience to Yahweh yet most "Christians" are in rebellion against Yahweh.  These false "christians" justify their rebellion by stating that obedience is "trying to do works for your salvation".  They make all sorts of claims that "we do not need to obey the law because that is works and salvation is by graaaace" and "the law was nailed to the cross" and "the law is for the Jews"  These are the people verses21-23 is speaking of.  Messiah Yeshua clearly makes a distinction of doing works for salvation without obedience and obedience yet the rebellious claims that obedience is "works".  Yeshua also says the road is NARROW and FEW find it - most want fire insurance (salvation) from hell but few want to OBEY.  You must accept Yeshua as Messiah (meaning LORD as well as Savior) meaning you must follow / OBEY  Him otherwise He will reject you.  Yeshua is very clear that we are to obey and He will reject those who don't.

Matthew 7
13  Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:
14  Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it. 

 Scripture is clear that obedience is better than sacrifice (works) and (like Yeshua does in the above passage) the Old Testament also distinguishes between the two:

1 Samuel 15:22  And Samuel said, Hath the LORD as great delight in burnt offerings and sacrifices, as in obeying the voice of the LORD? Behold, to obey is better than sacrifice, and to hearken than the fat of rams.

Other verses in the New Testament that state we are to OBEY Yahweh's instructions (IE: The Torah or "The Law")

John14:15  If ye love me, keep my commandments.

John14:23  Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.

John15:10  If ye keep my commandments, ye shall abide in my love; even as I have kept my Father's commandments, and abide in his love.

1 John 5
2  By this we know that we love the children of God, when we love God, and keep his commandments. 
3  For this is the love of God, that we keep his commandments: and his commandments are not grievous. 

In the book of Revelation,  it also clearly states that obedience to Torah is as important as believing Yeshua is the Messiah (after all, even satan believes Yeshua is Messiah but he is not one of Yahweh's Holy angels - he is in rebellion with no chance to repent)

Revelation 14:12  Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.