You need a FireWire cable and power adapter to charge it.
Cable: https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=iPod+FireWire+to+dock+connector+cable&_sacat=0&LH_TitleDesc=0&_odkw=ipod+firewire+cable&_osacat=0&LH_TitleDesc=0
Power adapter: https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=iPod+FireWire+power+adapter&_sacat=0
Note: Some eBay listings already include both things in them, since they are used together.