Which class?
What do you hope to major in?
How many APs does your HS have? If they have 20 and you took 1, that does not look great…but there are many colleges for which that is fine. If they have 3 and you took 1 then that is another story.
If you do drop a class, you will have to notify the colleges you applied to because now your story has changed. If you cannot hack an AP class, will you be able to hack college classes?
Talk to your GC about what they recommend.
You need to change what you are doing for studying:
http://talk.collegeconfidential.com/high-school-life/2026961-what-to-do-when-you-arent-doing-well-in-a-hs-class-p1.html