Here is the full list of all 45 The Forest achievements. It takes around 50-60 hours to unlock all of the achievements on Windows.
In Single Player, don't kill any cannibals for more than 10 days in a row
Survive 5 nights
Eat an enemy
Kill bird with arrow
Revive 10 co-op players
Finish Game
Share a weapon in MP
Share a food or drink item in MP
Kill 6 or more fish with a stick of dynamite. Alone in Single Player or together in Multiplayer.
Eat all mushroom types
Revive one co-op player
Use the compass
Set off 20 bombs
Make and wear stealth armour
Cut down 100 trees
Knock down 6 or more trees with 1 explosive. Alone in Single Player or together in Multiplayer.
Collect all cassette tapes!
Survive 1 night
Sleep with another player (group sleep)
Eat an entire family
Craft all items
Kill all animal types and display heads
Grow all plant types
Play through entire game without killing or eating animals. Alone in Single Player or together in Multiplayer.
Craft 10 medicine items
Sleep on yacht
Explore all caves
Killed downed enemy with rock
Build a super long wall
Chop up 50 bodies
In Single Player, kill 100 cannibals
Find all missing passengers
Kill shark. Alone in Single Player or together in Multiplayer.
Kill a bunny. Alone in Single Player or together in Multiplayer.
Pass 50,000 steps
Repair a shelter
Collect all robot pieces
Cut down 1000 trees
Catch fish with a trap. Alone in Single Player or together in Multiplayer.