Making your account secure starts with choosing a strong password. Circle recommends that your password be at least 8 characters long. Circle allows, but does not require, upper and lower case letters, digits and special characters. Circle highly recommends that your Circle password be different from all other passwords you use on other web sites and that you use a password manager like 1Password or LastPass to store your password.
Circle requires 2-factor authentication on all accounts. If you are a Circle Pay user, Circle recommends that you configure your account to use Google Authenticator for the 2nd factor rather than receiving SMS text messages.