Cook's Assistant is one of the first quests ever added to OSRS, and it's super easy. All you have to do is bring three items to the Chef in Lumbridge. There are no requirements, and everything you need--including the quest itself--is in Lumbridge. You can finish the whole thing without leaving there. It takes roughly 3 minutes to complete if you are an ironman since you have to obtain all required items. Normal accounts can complete this quest within a minute.
You only need about 3GP to buy pot and bucket to get pot of flour and bucket of milk respectively. Eggs can be picked up in the Lumbridge farm.
General Questing recommendations: Enable RuneLite Quest Helper in your RuneLite plugin section, hold your space bar to skip through NPC dialog, and use number keys for chatting dialog.
Time to complete: Cook's assistant can be completed in two to three minutes or in seconds if you have all items beforehand.
Normal accounts can purchase the above required items from the Grand Exchange and talk to the Chef in Lumbridge Castle to complete the quest in seconds. Ironman must obtain required items from the locations shown below.





Completing this quest unlocks the cook's range in the Lumbridge kitchen which has lower burning rate. Some good next quests to do are Sheep Shearer, Imp Catcher, and Romeo & Juliet, as they are quick, provide useful early-game rewards, and are required to do some other quests in the game.