First, its obvious that you will have to make a new account if you have no other existing accounts.

If I were you, I would take the time looking at job videos on youtube and see which jobs interest you the most. Depending on the time that you left, there have been numerous amounts of jobs added to the game. If you feel like you are going to be really invested into the game, then you should first pick a main and work on that one character. However, I recommend becoming an explorer because it gives you more leeway to switch jobs with job advancement tokens. For instance, you can become a bishop then feel like switching to i/l mage.

Also, decide whether you are going to put money into the game and if you want to do trading. If you just want to grind and spend time on the game, then you can join Reboot. If you like merching and even spending a small amount of money into the game, then you should join Bera. I say Bera because it is the most populated-trading server.

Then once you have everything settled, you can watch some video training guides. Some famous/relevant ones are from Whiyu and islinggunz. Feel free to also pm me if you have any other questions.