Get to level 3-4: Kill chickens for feathers and sell them at the GE. They sell for about 7GP each. You don't need to do this a lot, as it is just for a little money. You can find them at the chicken coop just north of the cows.
Move up to levels 4-26: Kill cows, take the cowhides and bank them. They sell for about 200gp. DO NOT SELL THEM! You won't need armor yet. When you have a decent amount (about 140), get the money from the feathers and receive 10GP, then go through the tollgate to Al-kharid. Then to the bank, get out all your money and 27 cowhides. Next, go to the tanner and tan them for 1 coin each. This will make the price go up quite a bit! You can find the cows when you leave Lumbridge Castle. You will see a bridge, cross it and go North.
Move up to levels 26-55: Kill Hill-giants! You should have some good armor by this time. They drop big bones which are worth around 500GP! However, getting to them is a bit complicated. First you need to buy a brass key. Go out the south entrance of the G.E. and go West. When you get come across the bridge to Barbarian Village do not cross it, but go North. Soon you will see a small building with a ladder. You will need the brass key to go through the door. Enter the building and go down the ladder. Then you are there.
Move up to Levels 55-70: Now you have a choice. You can either carry on killing Hill giants or start with Hobgoblins. They can drop Limpwurt roots. They are worth 2k each! But they do not drop them every time. The Hobgoblins are found just north of the Hill giants.
Move to Levels 70-100: Now you have a bigger choice. You can kill lesser demons which can drop rune med helms or you can kill cockroach soldiers which can drop rune scimitars and rune square shield. Both of these do not drop these items every time you kill them. Lesser demons can be found at Crandor Island. There are other places but i do not recommend them. You can find Cockroach soldiers just north of barbarian village but south from edgeville. You will see a classroom with a trapdoor connected to it. Go down it and then pull back the poster.
Move up to Levels 100-126 Kill greater demons. They can drop rune full helms and rune maces. They can be found in deep wilderness and the dungeon with elvarg in it.