Mark 3

"Then He appointed twelve, that they might be with Him and that He might send them out to preach, and to have power to heal sicknesses and to cast out demons: Simon, to whom He gave the name Peter; James the son of Zebedee and John the brother of James, to whom He gave the name Boanerges, that is, “Sons of Thunder”; Andrew, Philip, Bartholomew, Matthew, Thomas, James the son of Alphaeus, Thaddaeus, Simon the Cananite;  and Judas Iscariot, who also betrayed Him." vv.14-19

Read chapter 3.
The Lord had many disciples, more than just 12, but there were 12 He appointed as apostles.  These twelve were given special authority by Jesus and thus appointed apostles.  Here we see that Jesus commissioned them on two specific things: that they'd be with Him and that they'd go out and preach.  We too can learn from this in that we must both spend time with the Lord and also go and tell others about Him.  We should not just do one or the other, both are important and needed.  What good is it if we learn but never share our learning?  And how can we tell others about Him if we don't even know who He is?  
Then we have the names of the 12 apostles listed. If you compare the lists of the 12 apostles in each of the gospels you'll see some slight difference.  For the most part they're in the same order with a couple of changes, but also different names used for certain men. For example Thaddaeus is also called Judas (see Luke 6:16), but perhaps not to be confused with Judas Iscariot he's called by what seems to be his last name?  Make a list of the names mentioned and order given in each of the gospel accounts and see what differences you find and reasons why.  This is a good way to practice sitting and learning with the Lord and then going out to share what you've learned with someone!