The bigger clubs will have priority systems for buying tickets, especially for the big games. Man U, Liverpool, Arsenal are the big draws, so everyone and his dog wants tickets. You may need to have bought a minimum number of tickets for other matches first, in order to be eligible for any 'big' match tickets. Bear in mind that there will only be small number of tickets available for a Man U game, to the general public, after the season ticket holders, club members, corporate **** etc have got their share. Maybe just a few thousand. And you'll probably need to either queue up at the ground or on the 'phone for ages on the day they go on sale, to stand any chance of getting any.
Of course, ticket agencies are the easier option, but you'll pay a stupid amount. My mate's wife paid £400 for a ticket for him at Old Trafford, against Aston Villa. Just a league game too.