Security is as paramount with bitcoin as it is with your personal bank account. The more secure you make access to your bitcoins, the less likely somebody will succeed in nabbing them. When asked to provide a password, for example, make sure it is unique. Don't use any password that you use on any other website, in case that website is compromised.
When using any online service, look out for additional security such as 2FA, which stands for two-factor authentication. With 2FA, even if somebody else discovers your password, they would also need to gain access to the second-level password which normally is reset every 20–30 seconds using a device such as a smartphone. Always immediately enable this additional security feature if offered.