It is not easy to start out with, but the more you stand for Jesus the easier it becomes.  You may want to just not say anything to start with, but with time, you will find that you will not be able to keep from saying something.  And most of all put yourself in Jesus' hands and He will put the words in your mouth.  Always depend on Jesus or you will foul up every time.  First you have to pray for the person, that the Holy Spirit will work to open that persons heart to what you share with them.  That's all that you can do.  From there the Holy Spirit will point that person to Jesus Christ.  We must realize that we ourselves can't save anyone, we just plant the seed.  You may never know if what you shared has any effect on that person, but just knowing that what you did was right thing will make you want to share with someone else.  The greatest feeling you will ever have is seeing someone come to Christ and realizing you planted the seed.  Knowing in your heart that this person will not spend eternity in hell.  If you would happen to see this shared testimony through and see the other person saved, don't get filled with pride.  Remember pride comes before a fall.  And also remember when you do share, don't talk down to them, be humble and just state whatever you feel that Jesus is leading you to say.  Don't get mad and lose you temper if they reject what you say, just knowing that you tried is what matters.