Hi Goatlady! Welcome to Postcrossing! 
Although the most common postcard size is 4x6", there’s a lot of variation in sizes out there, and sometimes you can even find postcards that are not rectangular! Anything is ok though — you don’t need to worry about this.
If you want, you can collect postcards, but that is not what Postcrossing is about. The goal of the project is just to connect random people through postcards — any postcards!
This varies from person to person. Some people have a lot of stock ready to send, while others just keep a few postcards from their town to send out now and then. You do not have to get postcards that fit specific member’s wishes — any postcard that you like or have available is ok.
It would be better to ask this question in the North America community, as the members in your country will have a better idea of what is normal there. Generally, I’d say most postcards would be anywhere from 20 cents to 1 dollar. Usually, buying postcard boxes (see some here, they’re relatively easy to buy online) is cheaper than buying postcards one by one.
Check out this topic for lots of inspiration!
Any kind of stamps are good to use in Postcrossing! If you’d like to simplify things, USPS has international forever stamps. But you can also combine other stamps to make up the right postage (currently $1.50).
I hope that helps! 
Please don’t stress out too much about the details — just enjoying sending a postcard to a random person somewhere in the world, and receiving some nice mail back!