Question about heels!

I'm sorry if this is a dumb question, but I don't really know how shoes should fit. Specifically, I mean close toed heels.

Should you purchase them a little bit tighter, or should you just be comfortable with them at the time of purchase? I've read on the internet that there should be a little bit of a slip while you're walking, but I don't know (or think) it's true.

Is it also recommended to have a platform for the toe? I'm browsing Reddit for tips to help break in painful heels, and make them more comfortable, but if there's any off the top of your head that you'd like to provide with me, I'd be very gracious.

Thank you so much, FFA! You guys have been extremely helpful for the short amount of time I've been here for. (: