Who are you witnessing to? Your family, your friends, or just strangers.
I ask that only because different people need different approaches.
One thing to remember is that it is the Holy Spirit that convicts, we are just the messenger. But for the Holy Spirit to use us effectly, we must have our hearts clean before God (that is confess and repent of any unconfessed sins) and we must immerse ourselves in prayer. We ought to pray for the Holy Spirit power to speak the gospel effectively and pray for compassion for those that we are witnessing to, pleading God for their salvation. As for being nervous and scared, the disciples prayed for boldness in Acts 4:31:
"And when they had prayed, the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and they spake the word of God with boldness."
In preparation, it helps if you have the plan of salvation memorized. I usually go with the Romans Road, which consisted of just 5 verses:
"As it is written, There is none righteous, no, not one:" - Romans 3:10
"For all have sinned, and come short of the glory of God;" - Romans 3:23
"For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord." - Romans 6:23
"But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us." - Romans 5:8
"For whosoever shall call upon the name of the Lord shall be saved." - Romans 10:13
I use these to explain four important points:
1) We are sinners - (Romans 3:10, 23)
2) Our sin leads to death, spiritually and physically - (Romans 6:23)
3) Christ died for our sins - (Romans 6:23, 5:8)
4) We must call upon Him (Jesus) to save us - (Romans 10:13)
If we would memorize these fairly easy five verses, then we will be better equipt to explain the Gospel and will know what to say, instead of stumbling over our own words.
Lastly, your own testamony is powerful. Tell others how God saved you.
God bless you as your share Jesus. I'll pray for you.
Love & grace
Dondi