OP, it used to be skinny, meek, and got picked on/beat up a lot, so I know where you're coming from. I think there's 3 components to it: physical fitness, actual training, and mental attitude.
There's millions of plans out there for you to choose from. I like a mix of powerlifting, work with weights to strengthen mobility and overall strength and flexibility, and cardio, but that's up to you. I think the biggest thing to focus on is cardio. You don't have to be strong to win a fight, but you will need endurance. Ideally you'd train both long-range (think running 5 miles) AND intense but short training (think sprint cycling). Essentially you want to train your lungs to be able to hold more air/ use it better so you don't get winded fast. For strength, I'd say to focus on more compound movements, and especially more "crossfit-style" stuff, like kettle bell work. This will help to build full body strength as well as balance and use of your body.
Pick a discipline, learn it, practice it, and stick with it. The most popular ones are brazilian jiu jitsu for ground work and either boxing, kickboxing, or muy thai for striking. What's important is to learn how to use your body, not your strength, to deliver blows.
You'll develop this from doing the former two. Learn to walk with your chest out, learn to handle verbal confrontation, and trust that you can fall back on your skills if those fail.