Come out of Chortlon St bus station onto Chortlon St the turn left up to the main road (Portland St) then cross over the road and go right past and you'll come to Picadilly Gardens. There's a bus terminus and you will see the tram stop right in front of you. You buy tickets from machines on the platform so some loose change is advised as they are beggars with notes.
The Altricham line will go through Old Trafford, you can also get off at Salford Quays for the football, both are close enough to walk to the grounds and you should be able to just follow the crowds.
The trams do get very packed on match days though (Picadilly Station which is about 5 mins walk from the coach station - you can hop on the orange number 1 free city bus outside the coach station which will take you straight there) is the first stop so you can try and get on there) with all the people trying to get to the grounds so it may be an idea to get up there early as there are plenty of places round Salford Quays etc.. you can grab something to eat.
http://www.thequays.org.uk/wheretoeat/ More options on the menu bar to the left.
There are fast food huts and such around the ground and pubs but be careful where you go.
A taxi wouldn't cost much but the roads and streets will be packed before and after the match so again earlier is best.
I live in Manchester so if you need any more directions or advice let me know, will keep and eye on the thread.