If you are 13 as you say then a ticket will be about £16 or £17 i think
not sure how many euros that is
a adult ticket is about £35
if you want a good atmosphere sit in the gallowgate end or the leazes end level 7
just stay in a b&b or a travelodge as they are pretty cheap per night
go on www.nufc.co.uk for more info on tickets