How to choose fresh, ripe (hot) jalapeños?
First, Jalapenos do not ripen once picked.  No pepper does.

Red jalapenos are actually ripe and have more flavor, although they are not, in my experience, any hotter than the standard green, slightly-less-than-ripe, jalapenos.  

There is no relationship between shape and capsaicin content that I know of.  So, mostly you're just trying to get jalapenos which are as freshly-picked as possible: no bruised spots, no spots or mold, stems not shriveled, etc.

Also, note that the seeds and membrane inside the jalapeno contain most of its capsaicin, so if you're looking for hot, do not remove those.