Welcome to the forums!
You need to post full hardware and software information as
per forum rules. We have to know what configurations and hardware you have in order to offer more relevant advice.
FreeNAS can be a little bit of a steep learning curve, but you really should put in as much time as you can understanding (at the very least, on a basic level) how the OS works and handles/interacts with hardware (drives, boot drives, networking cards, etc.). There are various other NAS OSes out there are simpler and easier to manage than FreeNAS. This is why I think you found in your search people saying "do more research" because there is a higher level of expectation for anyone looking to play with FreeNAS.
To start to answer your questions....I would do more research. I'll help you a bit, here are essential things you should read and understand to the best of your ability. If after reading through them, taking notes if you must, you still have questions then come back and post them so we can help you.
FreeNAS Manual
Noob Slideshow Explaining about Very Basic but Very Important Information Regarding FreeNAS
ZFS Primer
PSU Sizing Guide
Settings up Shares
Here is item by item responding with some added suggestions we need you to post in order to help you.
I installed Freenas 9.1 from a USB drive
Do you mean 9.10? Login to your GUI, Go to System -> Information -> Build then post your exact build of the OS you're using.
I am trying to set this NAS up for Plex and mass-data/media storage (videos, pictures, BlueIris camera recordings). I was following a tutorial where I don't believe I set it up correct and have a bunch of different guests and also CIFS doesnt work so I can't see the NAS on my windows network.
This is like 10 problems rolled into a couple of sentences. I recommend you start separate threads addressing those specific issues. You are less likely to get help because you are asking way too much at one time. Break it down and tackle one thing at a time in separate threads.
However, I will point you to this very straightforward and excellent thread for setting up plex. I have rebuilt my plex jail multiple times and it has worked without fail every time. Bookmark it and come back to it once you have your other drives added:
Setting up Plex Within a Jail
I am probably going to add 2 more WD Red drives (only have 3 at the moment and an SSD boot drive)
What I am looking to do is just start over from scratch. How can I achieve this in the easiest manner?
Ahh, now we are getting to the real and specific ask of this thread. People might chime in with different options about how to proceed but here is how I would go about it.
1) Backup your data - If you have any data on the drives right now, make sure you have it backed up to another location.
2) Reinstall FreeNAS on your SSD - I think it would be waaaaayy easier to just start fresh then try and troubleshoot or fix what yo have already attempted to setup.
3) Plan your new drive layout - Think about what type of pool layout you want (raidz1 [not recommended], raidz2, raidz3, 2-way mirror, 3-way mirror, number of vdevs, etc.). I'm not going to explain any of this, read the noob slideshow referenced above.
4) Setup pool with desired pool layout - this will erase all data on the drives you preiovusly had in the system, so before doing this, MAKE SURE YOU HAVE YOUR DATA BACKED UP!!!
This seems like a lot, but just take it step by step. I can tell you from personal experience and from reading others' stores on here, people that rush setting up with boxes always pay a price later, and we (the FreeNAS community) really don't want that to happen to you!
Good luck!