WHmcs will be recommended a lot, theres a lot other client billing system for webhosting , here are a few. Blesta , Clientexec , HostBillapp. Those I believe are all paid subscriptions. There are also free ones but not as good as the paid apps would be.
Best of luck finding the right one for you. Feel free to ask any questions about any billing systems here Ill try my best to answer them as accurate as I can for you.