You need a 6-7-8 speed chain from any reputable brand (preferably with a quick link), a chain tool, and time.
You'll usually need to shorten a new chain.
Plenty of youtube videos out there.
Remove old chain - quick link or push a pin out with chain tool.
Shorten chain to match old chain length with chain tool.
Install new chain feeding it through rear derailleur correctly and closing with quick link
Ride it and see if anything else is worn enough to not play nicely with the new chain. If it plays well together - Done
If it doesn't, you'll likely need to replace the cassette/freewheel which will require 1 more tool to remove/replace. Chainrings do not wear nearly as fast as rear cogs.