First of all, TV by the Numbers has put together a ranking system (based on this season’s ratings so far) that calculates which shows are most likely to get the axe or get renewed. On the list of scripted series, HIMYM is the fifteenth most likely to be renewed (alternately, the 29th most likely to be cancelled).
And if you’d like to see how How I Met Your Mother is doing now compared to this time last season (or the year before, or the year before), have some viewership charts:
Missing columns (2×02, 1×07, 1×14-1×22) are episodes whose ratings info I couldn’t find. Obviously ratings are far more complicated than total viewers, but it’s a figure that’s easy to find, easy to chart, and easy to understand even if you know nothing about ratings.