2002 Buick Century battery dead again

My Buick was dead last winter because the battery needed be changed. But the new battery only lasted for 6 month, it was dead again. We changed the battery in last summer. Unfortunately, this battery was dead again today. Before this happened, sometimes we needed to start the car 3-4 times. After it was started, it run great. Could you help me to figure out what parts we need to check or replace?

Thank you very much