Thats a good question and worth asking and thinking about.
The answer depends a great deal upon the other person and their circumstances. It depends upon their openness, their previous knowledge, how much time I had, whether I would see them again and so on. So I would approach a stranger on a bus differently to the way I would a close friend.
However given that, there are a few simple principles that you can use.
Pray. After all God is the one whose responsibility it is to convert them. So ask him for the right words to say and ask him to call the other person to himself.
Know the gospel. Peter calls us to “be prepared to give an answer to anyone who asks us to give the reason for the hope we have in heaven”. (1 Peter 3:15). So work out what you have put your hope in and why.
Get to know and understand the other person. Most people come into a knowledge of God through a relationship with a Christian. So get to know them and let them get to know you. By understanding them you can tailor your presentation of the gospel to pick them up from where thay are at and lead them to Christ. This is what Paul did in Athens in Acts 17.
Get them to read the bible The bible is the source of all our knowledge of God and is the tool that is used by the Spirit of God to convince people. Most people thn they knw what the bible says but few have actually read it. So try always to encourage a person to read the bible for themselves (usually a gospel like Mark or Luke) or better still try to get them to read it with you.
When they read the bible the aim is to get them to consider who they think Jesus is and what are they going to do about him. So these are the questions you need to gently enurage them to consider.
Be gentle and gracious as you seek to challenge them. This is the intent of Colossians 4:5-6. People do not respond well to conflict, so don’t think that winning arguments will win souls. Rather gentle and gracious telling people the truth will woo them into the kngdom.
Talk about your own experiences. This can be far less threatening and can also show that Christianity is livable.
Use other resources. There are plenty of resources available to help you bring the gospel to friends. There are small tracts, books, films and CDs that they can watch, read or listen to. It would be good to create a library of such material for yourself so you can lend them to friends. I suggest you visit:
for some suggestions. You could also encourage your friends to visit:
to investigate christianity and ask questions.
I hope this is helpful for you.