15 minutes is a really long time to meditate. Anyone who's new to it, I suggest starting with 30 seconds and work your way up to 15 in increments of 30 seconds to a minute. It's like starting with an empty bar when weightlifting.
In the shower, you can use the sound of the water or the feeling of the water on your head to anchor your mind. Since it's something you don't control and there are multiple sensory reactions created from it, it's pretty easy to be present and observe your thoughts.
This doesn't necessarily work for everyone but I have tried many types of meditations and this one is the easiest for me.
https://www.youtube.com/results?search_query=meditation+sri+...