How do I get 9,999,999 bells fast?

Get an action replay. Then you can have 999,999,999 bells in you're savings account

Action Relpay is the best. I hadn't been on for over 2 years, then just decided to pick it back up a few weeks ago and I had a full account. I believe you can collect interest as the months go by or at least I think you can seeing I had gotten more money.

Action replay isn't the way to go if "a" you don't like cheats or "b" you are playing on a 3DS.
Here are some easy quick ways and some longer ones.

Turnips:
Either buy large amounts of turnips from joan and wait to sell them to tom nook or
if you have another game card or know a friend who has the game ask them to help you out and split the profit. First you need to find a day where tom nook pays megga bells for turnips then get your friend or other ds with a game card and set the clock to sunday morning after 6am. Get your friend to open his gate and you can travel back and forth between the two towns and making money by buying turnips from joan at a low price and selling them for a high price to tom nook. The downsides are that you could loose a villager and if your friend sets his clock forward to the correct time he might loose villagers and his flowers might wilt.

Another way is to fish for sharks if in the summer they appear at night in the ocean and sell for huge amounts of bells and you can sell them online for even more once summer has ended.

You could go online and sell rare items online or hold games such as races or wrestling online and charge entrants and supporters. Don't forget offer a fair prize.

Other than that just keep collecting and selling. I got these from ign it's a walkthrough called survival guide.

I wanna pay of my house, I am 12 so I am not going to buy a hack, I really can't, I have tried the put money in the back and skip 100 years but I want to pay off the 798,000 bells of my house, it will take forever to do that interest chest, I want a cheat that will be quick and I won't have to pay real money

I'm not sure your purse will hold that amount but you can get 99.999 bells repeatedly by putting about 50.000 bells into your account (Try putting the time to 1/1/2000 and then put it forward exactly a year for 10.000 bells off your mom per new year, try it!) Then put the time back to 2000, then go forward to 2099 and you should have lots of money, repeat to become a billionaire! The cost of this is that you'll get LOADS of weeds so if you want your town to be perfect I reccommend you don't do this!

You guys are awesome thanks !.

Just set it to your birthday every year and you will be rich in Jo time!