I'm familiar with the following articles on this subject,
and recommend them to you ...
Does Praying Do Any Good?
... Why Should You Pray?
http://www.watchtower.org/library/w/2000/11/15/article_01.htm
The Power of Prayer
... God Does Answer Prayers
http://www.watchtower.org/library/w/2000/3/1/article_01.htm
Prayers That Are Heard by God
http://www.watchtower.org/library/g/2002/9/8a/article_01.htm
For more information, you can use the links to related articles (top-left of text), or utilize the index/topics/search pages. (Use these lists to turn the page to some of the sub-titles.)
Sincerely,
"Make sure of all things; hold fast to what is fine."
1 Thessalonians 5:21