Cot bed mattress

Rachel • 24, Hull UK 🇬🇧 Expecting baby number 1, 👶👑 my little princess due 29th February
I have a cot bed but no mattress. I've looked at mattresses and it's quite confusing, for me I'd just lay on one! 
Do I need a foam one a spring one an air sprung one? 
I don't want to spend loads of money on one but I don't want one too cheap that I have to keep replacing. 
Can anyone help please