Is MS's Win10 antivirus app good or do I need another one?

As long as you're not doing or downloading anything crazy it's good enough.

I personally would disable it and get a reall anti-virus like Avast,Avira,AVG,Bitdefender,Kaspersky,Panda Dome,or 360 Total Security. All 7 offer a free and very good version of their paid software. The Windows firewall is sufficient.

It is very good. I use it and have not been infected.

Windows Security as it now is (old name was Defender) is much better than its predecessor, though it is very slow to update when a new threat is found. It may be sufficient for many users, others prefer a more accurate and quicker a/v. You make your choice, it's up to you.

It is called Windows Security (old name was Defender).

It is a good Anti Virus program. I have been using it since Windows 10 first came out and have never had any problems with it.

I also use Malwarbytes (anti Malware Program)  (Free Version). It works will with Windows Security.