You multiclass either through feats or in addition further multiclassing in Paragon Path.
With feats, you take an initial feat that grants you; another trained skill, some manner of class feature or power that is usually per-encounter, and access to that classes paragon paths and feats.
You can then take additional feats, 3 in total to replace one of your powers with the multiclass; encounter, daily and utility of the same level or lower.
With Paragon Path multiclassing you gain (after taking all multiclass feats for that class):
-11th Level: Can choose to replace class at-will with multiclass at-will.
-11th Level: Can choose a 7th level or lower encounter power from multiclass.
-12th Level: Can choose a 10th level or lower utility power from multiclass.
-20th Level: Can choose a 19th level or lower daily power from multiclass.