Well, i'm assuming you mean tickets for the league game next year and not the Charity Shield as most have assumed. Obviously these probably aren't gonna go on general sale, so you'll need 2 One United memberships to enter the ballot which won't open till some point in march/april next year (you need one membership per ticket). If you're unsuccessful, you'll also have the opportunity to buy unused season tickets from viagogo.
Looking at your location, I'm guessing you'd be travelling from Ireland, so it's worth bearing in mind that this game will more than likely be moved to Sunday 8th May.