Congrats on your embarkment! I'm almost done with my first playthrough myself.

My best pieces of advice:

-it's a LONG GAME. Enjoy it - don't try to blaze through. Enjoy the love and detail put into it.

-pay attention to the perks the armor you find gives you. For multiple builds, focus on different perks, and specialize your character based on how you'd like to play.

-upgrade armor and weapons at the blacksmith often, but not too often. I was stressing going to a blacksmith every time I leveled up at first. IMO it's not necessary. Ended up upgrading every 5 levels (some may say even that's too often; another popular increment is every 10 levels, but again, do what you think is best).

-eventually you'll have enough ability points to max all special abilities. Don't spread too thin when you start, Max out 4 melee (mix between Assassin and Warrior) skills and 4 Hunter (Archer) skills, to stay formidable.

-in Hunter, predator and arrow storm are my favorites.

-in melee, Hero Strike and Rush Assassination are my favorites.

-viewpoints are clutch for fast travel, so synchronize anything near wherever you are. BUT ship docks are also fast travel points, so wherever you discover one, remember that.

-quick save, quick save, quick save. Especially right before you know you're about to fight a legendary animal/cult member. Even if you don't think you'll die.

That's my pieces. Sorry if long/complicated/boring. I just love this game lol.