Number 19, 23, 32, 34, 40, 62, 81, 87, 115, 271, 280, 492, 590, 982, 990 buses have stops near St. Peter's church and Castel Sant’Angelo. The closest metro stop to Ponte Sant'Angelo is Ottaviano (line A). I strongly suggest taking a cab.
Piazza Trilussa is one the most famous squares of Trastevere, in between Lungotevere Farnesina and Lungotevere Raffaello Sanzio, right opposite Ponte Sisto.