You can do a search on the internet or in the library for previous studies social scientists have done on how dress affects others' impressions. I'm sure there are tons of studies out there.

For your study, you'll want to come up with something more specific than "nice clothing makes people like someone better". Consider, for example, how clothing affects an employer's impression that the subject would be a good employee. Your hypothesis might be that an employer is more likely to hire an interviewee dressed in business attire (get specific here on what exactly that is) compared with someone dressed in say, a sweat suit. While you could have two test subjects actually dress in these outfits for the same interviews, you'll be introducing many variables here. Maybe the resumes they submit to the employer are different, maybe the employer enjoys the personality of one of them more, or maybe even the employer feels differently about the outfits at different times or days of the week. A better way to decrease the number of variables here would be to show photographs of the same test subjects wearing different outfits to potential employers, ask which they'd hire, and present your data that way.

If this is for the science fair, don't steal my ideas...I might need it!

There are at least two reasons.

First, how you dress indicates how much care you take for your appearance and how much respect you have for people you interact with by your effort to make yourself presentable.

Second, different clothing can contrast or fit the color and shape of your skin and body. This is particularly obvious with sexual interest: some clothes flatter a person's figure, some hide it, and others reveal the unflattering elements.

I suspect there have been experiments on the second point, but I'm not familiar with them.