how/where do i learn how to defend myself?

I'm a teenager, and I literally know nothing whatsoever about defending myself! I don't exactly want to be taking a white belt karate class with a bunch of 5-year-olds though.

I don't want to be a martial arts star or anything, but I want to be able to kick someone's ass when threatened, or if in trouble. I want to be able to protect a friend if someone is trying to kick their ass, and most definitely want to feel a lot more safe/secure than I am now.

The biggest problem is that I'm an absolute weakling. When my friend tried to teach me how to block, he practiced kicking and told me to block, but I failed, and when he kicked faster, I covered my face/head/specific areas, to which he punched (but didn't actually hit) my gut, because I left it open. Pretty much, when it came to it if someone was actually trying to hit me, and I used those blocking "skills", I would get my ass handed to me.

It's like whenever practice turns into full speed pressure, I die. In a hole. It feels like it's too late for me to develop these skills properly... I also just honestly don't know where to start. requesting help, I feel like a newb, so please help this potato newb.