Fake confidence in yourself and eventually you will be so used to acting confident that you will be. Try coming up with a mantra, or remind yourself of your favourite qualities about yourself. For example if I was feeling not attractive I might look in the mirror and tell myself that I have nice skin, or eyes or something. Though I guess some people wouldn't wanna talk to themselves in the mirror...you could just say it in your head =P
not everyone is attractive heck not a lot of people are attractive..it's all about confidence, if you have confidence that's the most attractive about a person. but if you want to do some things physically to build your confidence then go ahead..maybe pick out/wear your favorite clothes, maybe do your hair a little bit etc..but of course it's not necessary...everyone's attractive in their own ways
One way to feel attractive is to be healthy and have a lot of energy through a healthy diet and exercise. When your busy doing activities that you enjoy you will gain confidence and feel good about yourself, which all leads to feeling attractive.