No! noobs shouldn't be allowed to build anything. But since you are asking for advice don't build anything you personally don't need. Trust me there will be plenty of idiots on your team to choke the build limit with useless things or 4 corrals.
If you notice noone on your team playing defense in ctf then ya you should build up the defenses a little bit. But the best defense is skill with weapons not turrets and walls anyways.
Do not build the great wall of china around your base.
Check what others have built in base before deciding you need to build it, as there is a limit to the amount buildings you can have. 32 is the max. This includes walls.
MP has as steep learning curve, but bear with it.
Play, learn, enjoy.
Yeah the one thing that would have really sucked is if we couldn't be able to tear down some of the structures that go up. At least they made it possible for us to start destroying some of the more useless structures.
About building you will get it down after playing some matches. Really just build what you need. If I am close to the enemy base I will usually drop a bunker but just to get my weapons and then destroy it. Spamming bunkers can be more trouble for you and your teamates, also when you want to build somthing pay attention to the bottom of the screen and it will show you if there is already one of the structures nearby.