How do I treat a burn that blistered?

Keep it clean.. and prevent the blister from breaking... otherwise you open the path towards a possible infection. The fluid in it will dry soon enough. Resist any urge to pick at it during the healing process or you will increase the scarring.

If you start seeing puss or other indictaions of an infection... then you deal with that when it happens.

How bad is the burn? What did you burn yourself on? Some burns need medical attention, while others don't.

Of course... I was assuming it's a small burn with nothing more than a blister... as J_9 indicates... more exensive 2nd degree and all 3rd degree burns should be looked at by medical personnel.