While I’m not a pro, I can give you the basic gist. Basically you buy turnips for a low price (90-100 basically) and sell them for a profit, preferably anything over 100, or over what you bought the turnips for. I recently bough 26,000 turnips for 90 bells each and sold them for 500 each, and made about 1 million bells profit! The Official Turnip Prices thread in The Airport helps too, so you might want to check that out as well!