go to the Crafting Guild south-west of Falador, wearing your brown apron. You will be 
able to get inside. Then, mine gold ores until your inventory is full.
There are many players there so be sure to be in a less populated world.
After, go to Falador and put all the gold ores in the bank.
Go back to the Crafting Guild and repeat over and over again until you have at least over 
100 gold ores in your bank. Then go to the Grand Exchange and sell everything.