Not many people would expect an ignorant person to give a public speech. And should that person be brave enough to do it, he would end up looking foolish. Of course there are always exceptions.
Still, many pastors expect the church members to go soul winning without preparation. No wonder most Christians realizing their lack of preparation, give up soon.
Because pastors are busy in other matters, few teach their members how to witness or to give a simple invitation.
Knowing this, what is the average Christian to do? Well, he can learn how on his own. Some may think that it is not spiritual to learn how to witness; that it somehow diminishes the dependance on the Holy Spirit, but such is not the case.
There are different ways to learn, some evangelists suggest the swim or sink method. With the result that most Christians following this advice end belly up.
Others advise Let your light shine, which in reality means do nothing. And should a Christian life shine brighter than the sun, how is he suppose to explain his blinding radiance?
I believe the best method is Education & Application. This means that you should study first before jumping into the fray.
The best way to learn is to go with an experienced soul winner and play close attention to his style.
Another way to learn is to study a few books on witnessing, and then go practice on a couple of people. You don't need to study for years, before you begin. A couple of months will be sufficient to get you started. Then start practicing what you have learned while it is fresh in your mind.
Start with the easy stuff first. Things like giving an invitation, or passing out fliers, which don't require that you speak a lot. Later on, as you gain more confidence, you will be able to start witnessing.
As you begin witnessing, you will make mistakes, don't let that bother you; instead learn from those mistakes. (I have been an Evangelist for many years and I still manage to stick my foot in my mouth)
Just make sure your witnessing doesn't become mechanical, or a sales pitch.
With time you will gain more education and experience, which will then allow you to engage and persuade atheists, and people from other religions.
Keep on learning about soul winning. If you don't, you will eventually become stale and ineffective.