What is a christian?

Areopagus, or Mars Hill, in Athens
The Areopagus, in Athens, where the apostle Paul made his famous speech in which he said, “Now what you worship as something unknown I am going to proclaim to you. The God who made the world and everything in it is the Lord of heaven and earth and does not live in temples built by hands.” (Acts 17:23-24).

Some say that it's a good person. Some say it's a devout person who goes to church every Sunday.

It would be interesting to see what the Bible itself says about this word. The word ‘christian’ occurs three times in the New Testament -

From these references, we see the word was a term coined by outside observers, referring to those who were ‘disciples’ of Christ.

And who were they? - The Greek word for ‘disciple’ literally means, a student, a learner. In Matthew chapter 28, verses 18-20 -

Jesus came to them and spoke to them, saying, “All authority has been given to me in heaven and on earth. Therefore go, and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I commanded you. Behold, I am with you always, even to the end of the age.” Amen.
Jesus designates a disciple as: one who is baptized, and who continues to learn Christ's teachings.

It is important to understand that the Bible isn't meaning a student in a passive mental sense, but rather one who puts His teachings into action in his or her life.

Hebrews 5:8-9 clearly shows that, to be right with Christ one needs to obey Him.

How does one start, then?