Browserssssssss

Hey,

I use to try and code quite a lot a while back but kept giving up because it wouldn't look right in either FF or IE... i know there is a load of other browsers too so how would i test the coding in all those browsers if i don't have them?

What sort of code would it be to align a box dead bang centre of the browser.. in all browsers?

Can someone write a little code for me to understand please?

+rep if u can thanks very much,
Jamie