Look up FARMHOUSE ales, FLEMISH ales, FLANDERS ales, BELGIAN SOURS, and LAMBICS in the recipe database. Farmhouse and Flemish ales are the easiest to make consistently, so you might want to start there.
If you've never had a sour before, you may want to go get a commercial example to make sure you like them before you make one. Most people think they taste like vinegar, but I think they are excellent! Some people will also just blend 1/4 to 1/2 of a sour batch with another beer (Reds and browns especially) to make a more moderate sour.
I'm also a big fan of sour Hefe's and Wheats (Gose, Traditional Bavarian Hefeweizen), but they are a bit less forgiving and hard to get consistent brewing results due to the lighter body.
My current favorite commercial example is Rodenbach Red, which is made by PALM Brewing and is a 1/4 sour blend Flanders Red.