Reddit mentions of Blue Team Field Manual (BTFM) (RTFM)

Don't bother with setting up things to try and attack. You will be on blue team and your job is defense. It's better to find someone to attack your VMs instead of trying it yourself.

A good starting point is just learning what the normal list of services look like on Windows and Linux boxes. Process monitor is a great tool for Windows to practice with.

Have a good understanding how ports work and how to lock them down. Be aware of the most common ports that are exploited, and how they're exploited. Don't let anything talk to other boxes that they don't need too.

Knowing firewalls is your best friend and will be the biggest factor in keeping your network safe. They usually use Juniper and Palo Alto firewalls.

Disable things like CMD and Powershell if you don't know how to use them efficiently yourself, because they will be used against you. It's better to get rid of it if you can't use it.

Avoid using the internet on the VM boxes because your connection can be intercepted and exploited. You are usually given outdated operating systems so be familiar with common exploits from yesteryears.

The blue team field manual is a great resource. It won't teach you anything but it's a very good reference point, and you can use that to dig deeper in other sources.

Honestly, you don't need to be a network security major to do well at these competitions. The red team isn't going to break everything to the point where it isn't usable or easily fixable until the very end of the competition. If you at least have some computer systems background you should be fine. Understand Windows/Linux, Active Directory, SQL, and Firewalls.

https://www.amazon.com/Blue-Team-Field-Manual-BTFM-ebook/dp/B077WF4WYV