When you unlock the second (or third depending on whether your loan was denied!) you will take out a loan as part of the story progression with the bank which you will repay on a monthly basis.
Make sure you pay the loan back on time, the game will prompt you if you want to pay early.
Once you have paid back the loan and included interest, this achievement will pop.
You will likely not finish the island before the loan is finished, so I would take it easy and breeze through the other achievements while you pay the loan off over time.