In Act 1, as all the scheming (on Cassius's part) takes place against Caesar, Cassius carries on conversations with Brutus and Casca about Caesar's rise in power and how he is not willing to endure being under Caesar's oppression. In the first part of the quote, Cassius is not literally referring to wearing his dagger, he means that he will find a place for the dagger--in Caesar. By ridding himself of his enemy and the one whom he feels is oppressing him (Caesar), he alone will be responsible for freeing himself. In other words, he is not going to depend on anyone else to secure his wishes or freedom for him.
At the end of this speech, Cassius says,
"If I know this, know all the world besides, / That part of tyranny that I do bear / I can shake off at pleasure" (1.3.104-106).
This is essentially a reiteration of Cassius's previous words--he will take care of himself, whenever he so desires.
These braggadocio comments on Cassius's part are ironic in light of his relentless pursuit to get Brutus involved in the plot (he "needs" Brutus) and his rather whiny, sheepish behavior in Act 4 of the play.