press esc and then keybindings. If you scroll down you can keybind all the marks you want. There are addons you can use aswell but i find them slow compared to simple keybindings. With keybinding it's easy to mark as you run from one pack to the next making it all go faster.
As for marking stuff i always mark skull and occationaly i can mark for x if there is 2 mobs i want to kill fast (runeshapers in hol etc). Most people like the tank to mark atleast one mob so atleast mark skull