In my opinion, to look good in front of your crush, first make sure to have good hygiene by showering and brushing your teeth every day. You can also make sure your hair is styled well to make you look good and wear a nice outfit that you are comfortable in. Be confident and use the correct body language as well as have fun and keep an open mind. See more details at wikihow.com/Look-Cute-in-Front-of-Your-Ex,-Crush,-or-Boyfriend-(Teen-Girls-Only).
Hey, if u really want to make a great apperence iron your shirt! haha oh and smell good! not that overpowering cologne smell though, just... nice. Girls love it when u smell good! hygiene and brushed teeth/hair defenately important! Just dont overdo the hair with too much gel. It's honestly more how u act than what you look like so Nkathers was right, wear what u feel comfy and good in, ur confidence and comfort will show thro more than uncomfy stuff that doesnt look good. oh and the age old clique, tell her she looks good, flattery will get you everywhere.
Coming from a girl, I like guys who kind of show off, have that cutsie Justin beiber-like hair, look and smile at me a lot, and actually talk and listen to me. Most of all, be yourself! I know probably lots of people have said this, but girls don't like guys pretending to be who they're not. Hope this helps! ;-) good luck!