Good question. Showing up for an interview can be stressful, and having to pick the right outfit is even more stressful. Despite the stress, I have learned a few things about how to properly dress for an interview from having interviewed for jobs at least 10 times.
For guys, suit and tie is the standard. It really depends on the job that you are applying for, but having a well fitted suit, nice tie, wrinkle free shirt, and shiny shoes is the standard that all guys should operate by when showing up to an interview. If you aren't working in an office or you are interviewing for a job that requires physical labor (i.e. farmer, fireman, forklift driver etc) then a suit/tie combination might me a little bit overkill. Guys, always remember to do your hair, shave, and brush your teeth; these little things make a huge difference when you are meeting your employer for the first time.
For girls (pardon my lack of familiarity as I am a man), a suit, nice dress, blouse+nice pants, and nice foot wear (think heels, nice flats, or wedges) are the standard. Every place has its own culture so you are going to have to judge for yourself what is the most appropriate to wear. Similar to the guys, little things go a long way so you likely do not want to show up with really messy hair. It is always better to play it safe and overdress than show up really under-dressed in tattered jeans and a T-shirt.
It's hard to say that you should "100% of the time wear X" as every employer has a different working culture and expectations. You will likely have to do some research and judge for yourself what is the proper attire to wear.
I hope this helps and I wish you the best of luck!!