Advice from the old lady...
Women notice grooming, smells and shoes first. I know I do. Shoes (and how to wear shoes that are appropriate for an occasion say so much about a man you wouldn't believe it.)
Style preferences are going to vary from man to man but let's start with a few attraction killers and basics. Be sure these things are up to snuff and you will have an improvement in your confidence based on your looks, and a corresponding improvement in your results.
Things that matter for immediate impact:
1. Pot bellies and beer bellies are a turn-off. They are ugly and scream "slob." The idea of seeing one naked is gross. If you have this issue you need to start eating better and working out pronto. Drink less if you need to.
2. Running shoes are for running ONLY. Never wear running shoes in a casual setting. Nothing screams "Stylishly inept" faster. If you have to have some sort of athletic shoe for casual wear get a pair of Vans or a pair of Sambas. Never ever running shoes. Tennis shoes are a close second unless you are playing tennis. Keep some good leather or canvas flip flops handy and keep your toenails clipped. No flip flops if you have toenail fungus or ugly feet (you know who you are.)
3. The amount of hair you have on your head does not matter. HOW you wear it does matter. If you have great hair, get a great haircut and comb it properly; if you have lost the top out of your hair keep it clipped short or shave it bald. Richard Nixon dos were not sexy then and they are not sexy now. Bruce Willis and Mr. Clean looks will score you the chicks. If you get flakes use a dandruff shampoo. Dandruff is not sexy.
4. Take care of your face. Grooming REALLY matters. If you wear facial hair, keep it trimmed neat. If you clean shave keep it shaved (unless its Sunday and you have sexy 2 day stubble. The guys with sexy 2 day stubble know who they are; they've been told by women.) If you are patchy, either wear a goatee or shave and do not do the 2-day stubble, you'll only look like you didn't bathe or didn't care (both bad). If you have acne use an exfoliant, get a little sun to help dry it out, wash you face with a gentle soap twice a day; cut back on dairy and sugar (which feed the bacteria that proliferates in acne.) See a dermatologist if its bad. Keep your hands OFF your face & wash your hand often. Dirt from your hands is a major contributor to acne.
5. Get some nice aftershave or cologne. Women remember smells. My first serious LTR wore Drakkar Noir. That was 25 years ago. I still recall his sexy smell.
6. Floss. Brush your tongue and gums as well as your teeth. Bad breath is like body odor. It is an instant attraction killer no matter how cute a guy is.
7. Plastic watches look cheap. They are also clunky. They are fine for a 5K or a workout, not for a night out. Ditto Fitbits and the like.
8. Keep your car clean & neat. Sloppy cars indicate a whole host of sloppy/lazy habits. Says all the wrong things about you.
9. Be able to make a decision. Indecisiveness is NOT sexy, even on really little things. If you can't/won't lead how can you expect her to follow? Make statements and lead by making the little decisions. She will tell you if she has a problem with something you say (food preference for example) and you can suggest an alternative at that point.
10. Learn to kiss well. If you are a good kisser she is going to want to discover what else you are good at. Kissing is a skill and there is stuff on the internet to help you figure that out.
That's kind of a grab bag, but each of these things matter. Some are simpler to fix than others, but this list will help you in the right direction.