If you don't have something like Extended Events already capturing your data, then you're completely dependent on what is currently in cache accessed through the Dynamic Management Views (DMVs). The problem with the DMVs is that they only show aggregate information. It won't show individual query calls. So while you can see which queries are currently in cache and you can see which queries have been called within the last 30 minutes, you won't be able to tell which of these queries is the highest cost within the last 30 minutes.

However, the place to start is sys.dm_exec_query_stats and then go from there.