What is the etiquette when it comes to homeless people?

I know I'll probably get a lot of rude and condescending answers regarding this post but I'm not sure what to do. I'm living in a big city and I've never really had to deal with homeless people before asking me for food or money and I'm not sure what I should do. Should I ignore them or politely say I'm sorry and keep walking? Is there a specific way to handle it?