Welcome to AndroidCentral Thanks for the question.
As the above member said, is there anything odd happening on your phone that makes you think it's been hacked or monitored?
There are 4 rules that I follow to ensure my phone always stays as protected as possible.
1) Always ensure you have the latest Software version installed. This includes the Android version and Security Patch version.
2) Always ensure that all App you download come from the PlayStore and have a decent rating or a fairly high number of downloads.
3) Don't leave your Bluetooth, NFC or Wi-Fi hotshot on if you are not using it.
4) Ensure your phone has a password to unlock. This stops people from accessing your private information and possibly installing apps without you knowing.
In addition to the above, if you are a person of interest to law enforcement agencies, you will never be aware that your device is compromised until the handcuffs come out... Just kidding.... About the handcuffs...
Since not even the NSA could prevent their servers from getting hacked, you don't stand a chance of total prevention, unless you go into airplane mode (and hope that someone doesn't develop a way to hack into a phone through the charger.
That said, follow Matty's advice (and for #2 , read the comments for the app) and install Malwarebytes Anti-Malware and a good anti-virus app.