First off all wrong,the first thing you should do is niche research and in the same time keyword research.Altrough you are running a PPC campaign(which is based on keywords),correct?
First search for "good" keywords with low competition because otherwise your competition will "eat you alive",you will be in a situation like biding $2 for one click.When you find good keywords with low competition(if there are any in that niche) see if the campaign can be profitable in that niche,ask people for honest answers on conversion in that niche(average conversion for all niches is 1:100),because conversions vary from one niche to the other and that can decide if your campaign will be profitable or not.
From the start you must know what you are going to sell,so you can do niche and keyword research for that service or product.
So you found a good niche,less competitive keywords and now is time to build your landing page and write a sales letter.(for writing sales letters i would recommend you to hire a native english)
It is always better to have your own landing page,Don't direct link to the merchant.