I'll cut right to the chase here and try to save you some money. If you don't have a solid understanding of keywords and Google's rationale for charging $9.36 versus $.05 for an Adwords block, go back and learn it.
You've heard it all too many times on forums; someone poured hundreds of dollars on an Adwords campaign that just couldn't fail. And yet, it did. Now, they are all over Google like white on rice, complaining that Adwords is crap and Google is a shady thief in the night that stole their money. Nothing could be further from the truth.
You CAN make money with Adwords. Thousands of people make their sole income from Adwords and consistently get the best ad placement for their budget. How do they succeed? It's not a matter of luck, its the tools you choose and how you use them.
How to find profitable keywords:
Hopefully, you aren't flying blind and you already have chosen a product that you are confident is marketable. If you are an affiliate of the product, your affiliate management site may even have lists of keywords and keyphrses that have been proven winners on Adwords. Keep in mind that those lists are useful, but probably also saturated and you need to think outside the box to find keywords that will draw substantial amounts of TARGETED traffic.
I use several different tools in my Adwords campaign creation. Many are free, some are paid but have made back more than what they cost. Google can be your best free keyword research tool. Think of it this way, you have a product or service, plug it into Google and you receive examples of the top Adwords campaigns that your competitors are running. Check out the sidebar ads when you Google your product. Do they use single keywords or are they using more targeted long tailed keyphrases to draw traffic? Once you have researched the competitors ads, its time to use some paid tools.
In my experience, the single most useful tool has been Micro Niche Finder. Hands down, if you can only afford one tool for Adwords research, this is it. New features are added periodically and its probably the most powerful piece of keyword research software you will ever use. You simply plug in a keyword or keyphrase and MNF will provide CPC, competition, and number of times the phrase has been searched. This alone allows you to evaluate phrases and discard those that simply don't draw traffic or have too many competitors, which will drive up your CPC.
The other tool I use is Keyword Analyzer. This is the crack for the Adwords addict. You simply plug in your competitors' site and KA will tell you the CPC bids your competitors are using. If a phrase isn't being used, KA will still tell you what you can expect to bid and you can track all of this information over a period of time. I suggest that you track for at least a month to monitor your competitor's progress or lack thereof. How will you know if a phrase has been effective for a competitor? Well, one of the foundations of advertising is to pull a campaign that isn't working and is costing money. If your competitor pulls a phrase, its likely it hasn't been effective in generating any traffic (or revenue). Congratulations, you just eliminated a phrase without losing money!