The best resource for newbies at Constant Content is their forum. Soooo much helpful information there! When I started there I remember making an evening out of reading old posts. I learned a lot!!
If you are trying for success by going after Public Requests you have to be VERY fast. As in the second you see it posted, you are writing it. These are tough to get and can lead to big frustration. The best ones to pursue are those that have value beyond that of the requester. For example, if someone is asking for a review of a particular website, that article is only worthwhile if the original requester buys it. However if someone is asking for articles about weddings, your article can get picked up by someone else down the line if the original requester doesn't buy it.
I don't know what it hot at Constant right now, I am a bit out of the loop at the moment, but you can study the recently sold articles for trends.
Really the best advice is to spend some time in their forum--friendly helpful folks there!