Choosing the right network switch/hub for a medium/large buisness

I need to select a hub or switch for a 300-500 user network.
I have a server set up and firewall running on a linux machine, the computers are spread across different sites (roughly 300 in one site and 100 in each of the others).
Now the server is runing from the main large building, should I just use a large (or multiple smaller) switches in the main server room and connect them all through there?
That is what I was thinking, but I'm not sure what switch would be applicable.
Any help appreciated!

(And as I want to be honest, this is for a uni project, I have got this far and set up all of the softwares, network configs and everything and am now stuck on something as small as choosing a switch hah)

Thanks =)