Before you learn the easiest way to make money ever (My opinion), here are the requirements:
Lvl. 30 mining
MEMBERSHIP
Complete Rune Mystries

This part is for members. Free to play requirements are:
Lvl. 1 mining
Complete Rune Mystries

For Members...
Head to Varrock. Take off ALL of your armor. ONLY bring a pickaxe. Head to the Rune Shop south of the east bank. Teleport to the caves by selecting the teleport option on the owner of the Rune Shop. Mine the huge gray rock. With 30 mining, you will mine pure essence instead of rune essence. Pure will sell for 84 gold per essence. mine about 500 and you get 45,000 coins!

For Free to Play...

Same thing, only mine rune essence. It's not as fast, but it makes a quick buck!