This is actually a very simple achievement. Most of the quests come from the story line, but you will have to keep an eye out for the side quests.
To find the quests that you still need to complete you will need to access the pause menu, and select the quest page, scroll down to read the quests you need to complete.
NOTE: Only quests you have unlocked will be listed here, you will have to find the quests before you can see them in this menu.
In each area the side quests are usually the same, you will have to find someone a card they are looking for, and find the 6 children who are hiding.
All you have to do to find these quests is find the people that want you to complete the quests. Find a kid that's hiding (they all are pretty easy to find) and it will start the hide and seek quest. The person looking for a card will be very easy to find because he will have a card over his head.
This achievement was very easy, I did it with no help from a guide on my first playthrough, so don't worry. Just make sure to fully explore all of the areas, and talk to anyone that looks like they may have something they need you to do.
The achievement will not unlock until you beat the game, seeing as one of the quests is tied into beating the final boss this makes sense.
Good luck :)
EDIT: If you miss a quest in a certain area you can get back through the sarcophagus designated for the area you need to get to. Thanks A Wise Ninja for the tip.