Selam aleykum
My - question to some learned persons over in the forum - What and how should I give him the answer ? Because again I will find this person and will try to give him a fitting reply - and this time I will not let him go , Inshah Allah.
I'm not a "learned person" but I'll answer your question nonetheless...
1. I would discourage going there. Giving dawah is great, on the metro, at work, whenever you have the opportunity. But be sure to know what those opportunities are. An opportunity is when somebody is listening. People that try to convert you to their faith are usually not listening. I would leave the wet works
(=going to a place of worship of a different faith to be thrown in front of the lions) for the scholars.
2. Some questions are traps. Try to read between the lines. When someone who's promoting his/her faith asks an "out of the blue" question like that, it's usually not because they are genuinely looking for an answer. So the safest approach here is to reply by questioning motives:
"Are you genuinely asking me because you are looking for the answer or are you just asking me because you want me to consider?"
In most cases, the reason for such a question is that the other party has a nice text-book answer they want to share with you. Questioning motives will force them to either lie, in which case you can catch them on their lies, or be honest in which case you can simply reply:
"I'm pondering the question you wanted me to consider, and I find a perfectly logical answer to it without having to question my own beliefs or without becoming intrigued in whatever answer you're trying to pitch me."
This "diffusion" could save both sides from a lot of frustration over an argument that is irrelevant either way.
Other then that I'd like to point out the danger of answering a question to which the answer is unknown. Consider the following three questions:
1. What are the 5 pillars of Islam?
2. What are the 4 forces in Physics?
3. What does a soul look like?
If I answer question #2, everybody will know I'm not getting my answer from the Qur'an. The answer to question 1 on the other hand, will be straightforward text-book answer. But what about 3? Always make sure, that the person you're talking to knows where your answer comes from when answering a question like number 3.
Good dawah is not:
"answering as many questions as possible" but rather:
"spreading as much information that is both correct and relevant as possible, ".
Good luck :)