Read the part thoroughly. At least one day before the audition, make a list, after you understand the situation and plot, of your character's most obvious "sartorial aspects." This is not necessarily an acting exercise (although it will help) but more a way to determine how this person dresses. Consider the genre of the film first. Is it a comedy? A serious drama? Think about geographical location, time of day, age, income level, education and other elements. For example, say you are reading for a drama about a grieving mother of 35 who works as a lawyer in New York City and has come to the police station on a weekday to answer questions about her dead child. You might list such aspects that will inform her clothing choices as: 1. professional, 2. dark, 3. subdued.
Open your closet and scan its contents. They should be organized by type of clothing: dresses, skirts, pants, shirts and sweaters. Pull out anything that leaps out at you in terms of color or type of clothing. For example, for your grieving mother you would pick a daytime professional outfit (even if she's not going back to her office, she owns mostly professional clothing) that is not cheerful and doesn't call attention to itself. You might choose pants or a skirt and blouse, or a dark suit. This character is not dressing for affect nor is she particularly concerned about how she looks, but would probably try to be neat in spite of her grief.
Try everything on, including the right undergarments and shoes. Study each outfit in a full length mirror and take note of what makes you feel most like the character. Discard the choice of anything that feels or looks uncomfortable or doesn't fit properly. Discard anything that's too obvious or "costume-y." You are dressing to suggest, not to take the place of the wardrobe designer.
Once you think you've found the perfect outfit, be critical. Ask these questions: Does it look like I'm trying too hard? Does it suggest the character without looking like a costume? Am I comfortable and can I move easily?
Now read or say the lines as the character and move as you will in the audition. If you will be sitting, do it in front of the mirror to see how the skirt or pants look in every position.
Once you are satisfied with the outfit, make sure it is clean and pressed. Hang it in a place where you can easily slip it on the next day for the audition in plenty of time, without last minute mending or ironing.