Manchester is a major hub so it's not difficult to get here. The main points of entry are the Manchester Airport and Piccadilly railway station.
Manchester also has several other railway stations including Victoria (in Salford), but they all terminate at Piccadilly for arrivals. There is a taxi rank on the lower level and the free link shuttle stops out on the main concourse as well as city busses which stop there as well.
Manchester airport is international as well as domestic, has three terminals and all the usual amenities including taxis and car rental services. The airport is serviced by the trains that take you into Piccadilly and if there is maintenance work being done on the rail line, they will provide you with a free bus into Piccadilly instead.
Chorlton Street bus station is the main port of entry for long distance busses/coaches. It's a short walk from Piccadilly station in the city centre. National Express busses are coming and going from here all the time and another more recent alternative is the Megabus service which came out with cheap cheap cheap routes to major cities for anywhere between 1 and 5 pounds per person. National Express now has some competitive excursion rates so check them both. National Express might be slightly more comfortable and they do have more destinations to pick from but if you're going to a major city from here, Megabus might be a viable alternative.