I pulled the both out and they were black, and the tips were worn down, so I put 2 new ones in. I just bought the bike, it is my first bike. I was wondering why this was? My friend says because the fuel is too rich, so don't use the choke, but I also heard to warm up the bike for a few minutes before each ride with choke on, so what should I do?

I just want to keep the bike in good maintenance.
Thanks