I would like to build a bot with below conversation, could you please advise how to do that? Do I need Forms? Can anyone show it to me?
User: I would like to learn about your service. Bot: Okay, Service A , Service B or C do you want to know? U: C Bot: Okay, what is your xxxx regarding C. U: It is yyyyyyyy Bot: Okay, so your yyyy is zzzzz.
Welcome to the forum 
As you said, forms are exactly what you need, You can find lots of tutorials on the Rasa YouTube Channel, like this one:
I highly recommend watching the whole playlist and checking out the Rasa Learning Center.
I don’t know if you will be helped how you expect in your case, but the best options will be revealed to you. Some time ago, Mortgage Broker Huddersfield helped me with my mortgage issues. I was new in the city and country in general. I had a relaxed position as site manager and wanted a better living place for my family. But when I saw 33 papers for my contract, I went doubtful. After visiting this company for advice, I discovered many details. I was like a newborn baby in an unknown world. Thankfully they changed my mind and offered a better deal and help.