Describe why abstinence is the best choice. be as detailed as possible

First, I think it's important you know that abstinence means to abstain (or completely refrain from) any sexual activity. 
This is one viewpoint that could easily be argued against.
Here's why a lot of schools of thought would claim its the best choice:
- not having sex is the only way to really guarantee that STDs and unwanted pregnancies won't occur
-STDs combined with pregnancies frequently produces offspring with special needs, and diseases
-condoms still have about a 2% rate of failure, so no matter what, you are still taking a risk by having sexual intercourse
-many religious systems value abstinence seeing it as more holy than engaging in sex

But it may be a good idea for you to address the counter-arguments: sex is frequently considered "good" for mental and physical health, but you could bring up this minor point and then dismiss it saying something along the lines of "while there are certainly benefits to engaging in sexual intercourse, the risks of having sex undoubtedly outweigh the benefits"
Hope that helps!