An attractive person is well groomed. Be neat, tidy and dressed for the occasion, whatever it may be. When in doubt, dress down. If your appearance says that you care about how you present yourself, it adds to your appeal.
This need not necessarily mean expensive clothes or elaborate preparations, just an effort to look the role you intend to play in any situation.
But grooming is also about our behaviour.
Avoid habits that people find disgraceful when in company. This includes digging your nose, scratching at pimples, biting nails, leaning in close on people when talking, overpowering perfumes, unsuitable clothing, never offering to pay your share, ........ These things are different from situation to situation, but paying attention to those around you should be enough to see the things that woule seem inappropriate in the company you are in.
While it would seem commonsense, there is an alarming number of people who don't even know when they do it, even when they find it offensive themselves.