Make sure you have created network password for the service provider router and firewall box is enabled , settings should be automatic .
if you are using air port express see this article Recommended settings for Wi-Fi routers and access points - Apple Support
If NAT is enabled in service provider router then in air port express keep the setting as off bridge mode as double NAT will not work .
See some articles Choosing good passwords in Mac OS X - Apple Support
macOS Sierra: Tips for creating secure passwords
macOS Sierra: Add a password hint
In system preferences > iCloud - Keep Back to My Mac as unchecked , in system preferences > sharing > uncheck all the settings ( screen sharing , remote login ....) .
Avoid using anti - virus , cleaners , unidentified apps in the system , and change all passwords like admin password , Apple ID , I cloud , wireless network password .
Keep your Mac clean by running Malware Bytes Anti - Malware for Mac .
Dont share mail address to unknowns and don't double click on suspicious links in mails see this article Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support