What the account actually gives you
Just a username between 4 and 10 characters, a valid email address and a password of at least 8 characters — that's the whole form. There's no payment step and no separate application: submit it and the account exists immediately.
The account you create here works on both clients: the Windows client and the mobile client connect to the same server, so your characters, items and guild are identical on either one. Nothing has to be linked or transferred between devices — see playing on mobile for the details.
The account, the client and the game itself cost nothing. Anything sold in the shop is a VIP perk or a cosmetic — nothing that decides a fight — so a new account starts on equal footing with everyone else.
Once your account exists, download the client for your platform, install it and log in with the same username and password. Character creation happens the first time you connect — no separate step here.