You should have a good haircut. A haircut that hides your bad features and accentuates your good ones. A lot of shorter hair styles are considered handsome rather than longer ones.
Have good posture. You should not hunch your back when you are sitting or standing. It can cause back problems and make you seem unsure.
You should smile whenever you can. Smiling is not some sign of weakness or softness. There are a lot of men who believe this to be true but it is not.
Practice good hygiene. You should take a shower every day and brush your teeth twice a day (once in the morning, once at night).
Always be yourself. Just breath, relax and have fun when you are with friends. You don't have to put on a gimmick or act like an idol. Just be calm and have fun while you are doing it.
Have a dress style. The style doesn't have to be what ever is popular but you should feel comfortable in what you are wearing.