For home games, you need an official United membership to buy tickets (cost £32). Most of the home games will be sold out by now but you might be able to find the odd ticket. The tickets for the big home games (Liverpool, city, Chelsea, Arsenal, Spurs(?)) can be obtained from entry to a ballot. Ballot opens when the KO and date are confirmed. For example you could apply for the City game now, tickets are assigned at random and as you can imagine there are lots of applications so your chances of getting one are slim.

Away games can only be purchased by premium season ticket holders so are very very hard to get.

As always, you can get a home or away ticket by paying way over face value at the ground on the day. Alternatively you can monitor the United ticket exchange twitter and try and grab some on there, but these are offered on a first come first served basis so you'd have to be quick.