I had a dream about s3x but I'm 15?

Suggest you read Shaunti Feldhahn's "For Couples Only" as a clean, guilt-cleansing, and clear look at romantic relationship.

As a teen you will find yourself inundated with dreams involving sex.  You will have all sorts of new experiences.  It's all okay, as your body is changing from a child's body to a teen's body to a adults body, and with these changes your hormone level goes off the chart.  So the feelings are new.  Just be aware that it is not all in your mind, as it is mostly due to the overload of hormones.