The best way to get strated is with the Popular Landing Pages report. There you can filter by niche, and optionally by timeframe if you want to know what is working right now.
For example, if your niche is Weight Loss, you can select “Niche = Weight Loss”, timeframe = “Month to date” to see what pags are being run right now. A landing page with lots of ads is likely to be working well.
Another way to find (and filter) landing pages is to go to the Landing Pages section and filter by different criteria.