Witch's Potion

Hetty the Witch
This is one of the easiest quests there is. You should have no problem with this, no matter what level you are.

Talk to Hetty and tell her you want to know more about the black arts. She will tell you she needs four ingredients you have to get - a rat's tail, a piece of burnt meat, an eye of newt, and an onion.

Rat's tail
You need to start the quest to get a rat's tail. Go west of Hetty's house and kill a rat in the ranged shop to obtain a rat's tail (only the level 1 rats drop the tail - you will have to kill a level 3 rat for the meat and a level 1 rat for the tail, although it may take a couple or so kills, so bare that in mind.)

Burnt meat
Kill a giant rat southwest of Port Sarim and cook it. There is a range marked by a pan on your map where you can use to cook the meat on just south of the magic shop in Port Sarim, logs also spawn in the general store of Rimmington. If you burn it, good. If not, use the cooked meat on the fire or range again to burn it. Or you can buy raw beef from Wyin's shop in Port Sarim.

Eye of newt
You can buy an eye of newt from the magic shop in Port Sarim, it is labelled with a small fire rune on your map.

Onion
Onions can be found in a field northwest of Rimmington or in the backyard of Farmer Fred's farmhouse west of the Sheep Pen in Lumbridge.

Finishing up
When you have all these items, talk to Hetty. She'll put them in her cauldron and tell you to drink from it. Drink from the cauldron to complete the quest and gain your reward.

Rewards

 * 1 Quest point
 * 325 Magic experience