What do i need to do to be saved?    

Hear the gospel, believe the gospel, repent of your sins, and be baptized for remission of sins.   gods-plan-for-your-salvation.html

when did the church of christ begin?

The Lord's church began in Acts 2.  Peter preached the first gospel sermon and told people what they needed to do to be saved. Click Here for more information.

What is sin?

The Scriptures define sin as "the transgression of the law" of God (1 John 3:4). A sin is something committed. Every person who is accountable for his/her own actions has sinned (Romans 3:10, 23). Some have said that they have never sinned. Well, just making that statement was a sin. The Bible says, “If we say that we have not sinned, we make him a liar, and his word is not in us” (1 John 1:10). Notice what this passage says, "...we make Him [God] a liar..." Well, we know that this is impossible, for it is impossible for God to lie (Hebrews 6:18).

can i be lost after i have obeyed the gospel?

There are lots of warnings to christians to remain faithful. There are examples (i.e. the Corinthians) warning us not to fall out of favor with God.  We must remain faithful after we obey the gospel if we want to reach heaven. 

do all churches of christ teach the same thing?

Unfortunately they do not.  Over the years some brethren have added to the scripture.  In order to  be pleasing to God we must "speak when the Bible speaks and be silent when the Bible is silent."