Watch the Modulating online guitar lesson by Matthieu Brandt from Songwriting on Guitar
There are many ways of switching to another tonal centre. Some introduce the new set of chords gradually, other ones will do this abruptly. The first candidates to madulate to are chords within the key other than the original tonic chord. For instance modulating from the tonic chord to the chord on the fourth or fifth degree. These are always 'near' modulations. Stating the new key can be done in an obvious manner (through a V-I progressions for instance) or more subtle.