It is very tough to be simple. Not every genius can do it. I don't know about looks, but what I can sense here is a desire to appear professional, smart, and capable. Right?
Guess as far as appearances are concerned, that can be changed—not necessarily by buying the best brand, but by ensuring that whatever you wear looks clean, smart, crisp, and fresh, be it your dress or your makeup (I don't know if it is applicable here? Your profile is not clear on gender). In dress, if you're comfortable with western wear, that is also good, but otherwise, even ethnic wear is great. But remember, nothing loud, nothing extravagant; the emphasis should be on staying simple. For inspiration, movies like "Corporate" and "Fashion" are good ideas to watch.
But all this is merely a background; the real thing will be the work you do and the kind of talk you engage in—here you will have to deliver. Without results, no impressions. The reason is simple enough—you have been hired to do a job! So keep your focus there—that is what will actually make you a professional.
ALL THE BEST!