Help!!! Roaches in the Summer time?

Hello..I live in Los Angeles, CA and I have roaches in the summer time. It's the big roach. Yuck!!! I found 3 within the last month. One in the kitchen, one in the bathroom and 1 in the hallway. I do not have a messy or dirty house however it is a old house. What can I do to prevent it? Also, this seems to happen during the summertime..someone help me.. It's disgusting and embarrassing.