i think everyone who comment will have a different opinion of what ''attractive'' is. you need to just stick to the basics. like good hygiene and being a healthy weight. muscles wouldn't hurt anyone...also maybe emphasize some of the things you love about yourself. maybe your eyes? ;) just don't try to change for other people. and don't get too narcissistic! everything isn't about looks.
Honestly, being yourself is usually the best thing, since there's so many different people who are attracted to different things. As long as you have good hygiene and do some kind of physical exercise, you're usually good. But one thing to avoid is "jeans so low you can see way too much of the underwear" look. It can look kind of tasteless.