>>212988781Short answer: Be interesting, put a modicum of effort into your appearance, be sociable. That's pretty much it.
Version for autists:
Get an active hobby (something you actually participate in as opposed to a passive hobby like watching tv and movies). Girls tend to like it when you are actively involved in a passion. It can be almost anything. Gardening, playing an instrument, woodworking, building models, cooking, photography, drawing, writing, keeping bees...just something that you can be passionate about that doesn't rely on other people entertaining you.
Hit the gym. There are things you can't control like your height or hair line or face, but being physically fit is something nearly anyone can do and people generally find that attractive. If you're out of shape, join a gym, download a workout plan, and stick to it. Wear clothes that fit well and keep them clean. You don't need to be crazy about fashion. A fresh pair of jeans that fit you, a clean plain t-shirt, and basic shoes that aren't falling apart are enough. Stay on top of hair cuts and if you have a beard or scruff, learn how to maintain it. Just learning how to comb facial hair and trim stray hairs makes the difference between rugged manliness and crazy hermit. If you are clean shaven, then make sure to stay on top of that. Scraggly shit hairs are not a good look. Also, I shouldn't have to say this, but shower and brush your teeth.
Learn basic etiquette. Be able to hold a conversation and limit how much you talk about yourself (you don't have to avoid it, but if you've been prattling on for awhile, throw the conversation back). Ask questions and actually give a shit about the answer. When you do talk about yourself, talk about things you like more than things you hate. If you struggle with socializing, there are books that can help guide you. "How to Win Friends and Influence People" is a classic. Practice with friends. If you don't have friends, practice with strangers.