Hello Muhammed and welcome to the forum,
You have touched the topics which need detailed explanation because there is no one line answer to that
I would suggest you look at this reference which explains how to choose best keywords list.
You can read the User articles mentioned in this section of the Community where Adwords Advertisers have written well informative articles along with their experiences.
Additionally, there are more aspects shared in this get started section.
Happy Reading and let us know if that helps!
Please note that growing business on Adwords is not an easy task as it looks to be. You have to give time and understand how the system works. If still you don't get to know this after spending good amount of time, you might consider hiring a Google Certified Partner for your Adwords account optimization. You can read this article if you want.
Still, if you think Adwords is not what you are looking for, you can cancel your account anytime and request the refund. For that, process is mentioned here.
Pankaj is absolutely right, to get a refund for any remaining funds you have on the account, you would need to cancel the account and request the refund. However, it is not possible to request a refund for moneys already spent on AdWords.
Hope that makes the situation a little clearer.