Aim for keyword phrases that have low-med competition rating. Then you can dominate the serps and make good money with your adsense sites.
PPC is pay per click. You're paying everytime your ad's link is clicked on, regardless of whether you sell anything or not. Getting tons of clicks is no guarantee of success, but it does show that your ad is interesting enough to make the reader click it.
In order to get traffic you need to, get a lot of backlinks to your site, raise its ranks in the serach engines. Each engine has a lot of factors enabling your site ranking. Relevancy, age, traffic, the right key words on your pages, correct image naming, article traffic, freshness (regular updates). Its a long process and easiest is to pay someone trustworthy.