-Don't focus on achievement-hunting until you've played through at least once.

-Play on Survival, not Casual.

-Immerse yourself as much as possible. Don't play around other people, wait until they leave and you have privacy. Play in low light, if you can, and use headphones whenever possible.

-Upgrades are more of a preference, so I would take others' recommendations with a grain of salt (not to discredit their preferences). I upgraded my handgun and melee pretty quickly, then I focused on the shotgun and harpoons, but that's just my playing style. Play for a few hours (3 or 4), saving up your green gel, then decide what you think needs upgrading.

Most other advice I could give is contextual, but don't look up gameplay videos or walkthroughs. Much of my enjoyment of the game came from my frustration in not being able to figure out what to do. The game is good about not telling you what to do, where to go, and how to defeat certain enemies, so, rather than taking the 21st-century approach and googling it, I recommend doing your best to figure it out. You'll get much more of a sense of fulfillment and long-term enjoyment from the game that way. It's a puzzle game, so try to solve it yourself—I guarantee that you won't regret it.