Unannounced visitors can be a hassle if business is busy, and hassling potential employers is not a good way to get a job. So call ahead before you visit. Explain that you would like to apply for a job. Find out when you should come in and who you should speak to. Ask about this when you are applying.
First impressions do count.
If you're applying for retail work, take time before you apply to notice how the sales staff dresses. If you're applying to lead sports for an after-school program, that will require a different kind of clothing. If you're applying to answer phones or do filing in an office, take time to think about what your potential co-workers will be wearing. No matter what the work environment is, always be neat and clean. It is also important to find out if the job will require special clothing or a uniform.