You have (2) options to power a Pi from your PoE switch (USB is not one of them):
PoE Hat: powers a Pi by just connecting the Ethernet cable to the Pi's Ethernet port; job done. Presently £20 (inc VAT) from all Pi retailers
PoE Splitter: has two connections: an Ethernet for data and a MicroUSB that diverts the power from the switch to the MicroUSB port cable that plugs into the Pi's standard power port. Presently £9.90 (inc VAT) on Amazon.co.uk
Both solutions work well, but one might be more suitable in respect to physical form factor form factor of the Pi's enclosure: the PoE hat adds a lot of bulk.
PoE Switch: The one I use- and recommend- is the ZyXEL GS1900-8HP-GB0102F
And FINALLY, when planning your PoE setup, ensure you chose an appropriate spec Ethernet cable. At LONGER distances you'll require SMALLER AWG cable. The LOWER the AWG number, the THICKER the conductor and LOWER the resistance. If you use a long thin cable to squirt the power long distances, you'll run into issues.
CAT 6 Ethernet Cables: I use these Videk 2996-3B CAT 6 AWG 24 3 meter cables and they work well. £4.06 (ex-VAT) From Farnell
Anyhoo, hope this little primer is enough to get you started down the PoE road.