By train
It's the cheapest and the best way to arrive in Guimarães. The Portuguese company CP (http://www.cp.pt) is the one that provides travel by train. If you come from Lisbon you have some, but few, direct travels to Guimarães. It's about four hours. From Oporto, you have a train every hour.
If you come by train from Europe, on InterRail, for example, you have always to go to Oporto first, and then you catch the train to Guimarães. This trip can give you the best attractions. One is the cheapest price, and the other is the wonderful views, passing over some small rivers, and small villages, in rural Portugal.
By bus
Rede Expressos (http://rede-expressos.pt), a national bus company, and is the best way to get to Guimarães. With this company you can travel from almost every big city in Portugal i.e. Lisbon, Oporto, Coimbra.
If you are traveling by cities near to Guimarães, like Braga, for example, or even Oporto, you always have to ask to people a very common thing in Portugal where you can get a bus to Guimarães you can try this in the cities within about 50km. You may make a lot of stops during the trip, but it's a way to better get to know the North.
By car
You have a lot of highways going to Guimarães, from Braga, Porto, Felgueiras, Chaves, and others. If you are coming from the south of Portugal there is no difficulty at all. The Portuguese highways are all very well connected, and you only have to go straight to Oporto, and then in Oporto you catch A7 Highway 7 which goes to Guimarães. It's quite easy.