Solid brass that is unplated will naturally darken/patina over time. If you want to bring it back to its shiniest self, you can polish it with a silver cloth or polishing pad. Don't put any chemicals on it or use any liquid dip cleaners (like those used for sterling silver).
Plated metals (such as 24k gold plating or silver rhodium plating) are amazing and need almost zero care to keep them shiny - just like a very forgiving houseplant. Just wipe with an untreated cloth and you're good to go.
To keep your jewelry in tip-top shape, avoid things like wearing it while skinny dipping in the lake, rubbing it down with lotion or sweating all over it at the gym. If you do one or all of these things, see tips above for how to deal.
In general, don’t throw it at the bottom of your purse or toss it on a hard floor or countertop. Always store in a cool, dry place and take it on the go safely in a jewelry bag or travel case. Treat it like it’s special and loved, and it will love you back for a long time to come.