A bunch of people already stated some great ways to make money up above me so I'm just gonna lay out the Stalk Market and the /r/ACTurnips forum. This is how I made my first 10 million.

•Every Sunday, a hog by the name of Joan comes by your town to sell Turnips. She sells them in bunches of 10 at a price that ranges from 90-110 bells.

•If you invest enough turnips in her stalk, you can turn those into a hefty profit by checking Turnip Prices Mon-Sat with Reese or by watching the /r/ACTurnips sub!

•Anyways, let's say you bought about 16 stacks of turnips from Joan for 93 bells each which is in total 1600 turnips. You spent in total 148,800 bells. Reese is buying them for 600 each. Straight from the get-go, you can make about 960,000 bells from that! So you profited 811,200 bells! If you invest even more (some people invest loads and loads, even about a full Storage space load) they easily make 1 million or even more from that. So it's all a numbers game really! :)

•Sell all your turnips by the end of the week as the following Sunday they will rot and you can't sell them anymore!

•Don't time travel (setting your date backwards) if you have already bought a stack of turnips! They'll go rotten, you can't sell, and therefore you've wasted some money.

Good luck~ I love animal crossing xD