Can you explain to me what an open call is? What should a person expect when going to one?
An open call is the time agencies set aside to meet with those interested in getting into modelling, without requiring an appointment.
The first thing you should expect is to be waiting for a while. If a lot of people show up you could be waiting a long time. Bring something to read and probably some juice or water. Even if you don't end up requiring them it shows the agency that you're the type that thinks ahead and comes prepared. If you're talking about an open call in Canada, I would be surprised if you were there more than 30 minutes. Most likely you'll spend less than 10 minutes at the agency.
Most agencies prefer no makeup, but there are some that prefer to see your sense of style. If you choose to wear makeup, keep it very light and fresh so the agents can see your true features.
Some people are paralysed at the thought of leaving the house with no makeup. If that's the case with you, there's no point in upsetting yourself. Just be prepared to remove your makeup when you arrive, then re-apply it afterwards.
The agency may choose to take some polaroids or digital snapshots, so wear something flattering to your figure. It might be jeans and a white t-shirt, or something more stylish, but don't hide yourself in your clothes. Don't wear anything too hoochie either, though.
Bring something to hold back your hair if it is long, and consider bringing a selection of pictures from natural settings (ones shot in natural light are best) to show off how well you photograph. If you can afford to leave some copies with them, they might appreciate that so they can show the other staff after you've left.