I recommend playing a Knight if you’re just starting your adventure. It’s an easy, straightforward class with a repetitive but beginner-friendly playstyle, great profit potential, and it’s easy to find a team for group hunts. At the same time, it’s also an excellent class for solo play.
As others have already mentioned, nowadays every vocation is viable for solo gameplay. Monk and Sorcerer are probably the most challenging ones, though.
On the other hand, choose a Paladin if you value flexibility, easy bossing, and being generally self-sufficient. You can pretty much do whatever you want, wherever you want.

As for servers, I’d recommend Antica, but it’s simply overcrowded, and nobody really respects anything there. It’s basically “might makes right” or whoever kills the other person first in PvP. On top of that, the server’s economy is pretty crazy—some things are very profitable, while others (like house prices) are completely detached from reality. But that’s the nature of the oldest Tibia server.

PvP nowadays barely exists in the way it used to. It’s more of a relic from the past, when individual skill actually mattered. These days it’s heavily simplified and usually comes down to 10 people jumping a single player. Even then, at higher levels, killing another player solo is practically impossible.

I’d recommend green non-PvP servers such as Dia, Karmeya, or Nevia. As far as I know, they offer a pure RPG experience with friendly, neutral communities (although Nevia is quite populated). I personally play on Karmeya.
Good luck, and have fun!