I have already been baptized in another church. If I become an Episcopalian, do I need to be re-baptized?

No. Once you have been baptized with water, in the name of the Trinity, you have been received by adoption into the family of Christ (not into a particular denomination) and that Sacrament is never knowingly repeated.  This is true even if you were a tiny baby when you were baptized. If you wish to make a public, adult, affirmation of faith, you may choose to be confirmed, if appropriate (see above).  You also always have the option of publicly reaffirming your baptismal vows, even after confirmation, if you so choose...but this is a highly personal matter, and not in any way required.