I'm a bit confused newbie

Hi, yesterday, I decided to test a DK in Wotlk PvP, I started looking for a private server and here I am...
I read the server description and understood that Blackrock is what I'm looking for. I don't really remember how the gear and season progressed back in a day so when people mention Ruthless, Deadly, Wrathful gear on forums and YT, I have no idea which one is better and which one was introduced in a particular season.

Anyway, my goal is to get the best possible gear to my newly created 80lvl DK and play PvP. I just have no clue how to reach that point.
I went around the area where my DK was originally placed, there are vendors that can sell me free enchants, general stuff, NPC where I can register an arena team, vendors selling PvE gear - I get it.

The thing is that my DK uses some Deadly Gladiator gear. Google says that it's S5 gear. The question is - what's the current season and how can I get (and where can I get) a next level gear?

There's also another very confusing part - there's a NPC that offers me something like skipping into S8? Alright, but how about the gear? I suppose I'll get smashed when I have only S5 gear?

Could someone please clarify this to me? Once again, my aim is to get the best possible gear as soon as possible and start playing PvP (mainly arenas). If I could just create an instant lvl80 character with the best possible gear, I'd do that, but I understand it's more complicated here and there are probably multiple options that I don't understand.

Thanks

previously Blacrock was "season 8" only. (which means Relentless and ToC gear is free, and you use Honor/Arena point for Wrathful and ICC/RS gear)

Recently, due to "Frostmourne seasonal" realm, new blackrock-characters are by defualt on "Frostmourne seasonal mode".
They have recently began "Season 7".
In this "mode" you play against "Frostmourne" players as well as other "seasonal blackrock" players

you have noticed that by talking to "Info and presets" Npc, you can enable "Season 8" mode. By doing that you will face "Icecrown", "Lordaeron" and "Season 8 Blackrock" players.

The majority of players are on "season 8" mode. I recommend to pick this mode
Enable it -> then talk again with that same npc to select your spec again (so he will give you a better gear)

Then by talking to other vendors you will see they have different items than earlier

Thanks, I feel like I'm actually getting somewhere. So when I switch to S8, I'll basically get a PvP gear that's 1 tier below and then I can play PvP in order to get the best possible gear, right? I suppose the vendor who sells the S8 gear is also located somewhere in the area under Dalaran so I can check the costs and other requirements?

the vendor are the same. Once you enable "S8 mode" they will show you different things.

You can create a new character as test -> enable S8 mode by talking with "info and preset" npc -> pick the spec ->
Then look at "Free gear" npc and "Endgame npc"

I have made a thread which may help you to understand how "Blackrock" realm works:
http://forum.warmane.com/showthread.php?t=322445 (I have not updated it to explain the recent implementation of "seasonal mode")