You probably don't want to hear this, but... image does matter. What you wear, how you communicate, your position, how you perform under pressure, all contribute to your effectiveness as a leader.
What you wear-
Society, as a whole, has become much more casual in what is considered as appropriate attire. Now that may be fine if you are attending an event, or working for someone else. But, as a leader, or someone who wants to be a leader, you should dress one level above what you expect everyone else will be wearing. If the group as a whole is in "business casual" (which often means jeans or khakis & a knit shirt), then your business casual should be crisp & put together (not a wrinkled shirt you just pulled out of your suitcase).
How you communicate-
Grammar & spelling count! If these are not your strong suit, be sure you have a good spell check and grammar check program for written communication, or someone who can proof for you. If you stumble over your words, practice (out loud) before you present.
Are you a "salesman" or a "consultant", a "manager" or a "team leader", "secretary" or "executive administrative assistant"? What makes you a "professional" in your field? If you get paid, you're a pro.
How you perform under pressure-
Do you crack & lose your cool or do you take a few minutes to assess and readjust? Do you blame others or do you take responsibility? Are you the problem or the solution? Would those around you agree?
Is it "fair" that people make immediate judgments about you in the first few seconds of meeting you? No... but they do (and you do it, too). So the question is... if you know this to be true, shouldn't you be aware of the image that you project? You will be more believable and more effective.