The Acorns app is a great way to start saving money. It's free to sign up and there are no minimums, so you can start with as little as $5. Acorns rounds up your purchases on linked credit cards and/or debit cards, then invests the spare change into an ETF portfolio of your choice. Acorns also has a "Found Money" feature that gives you $5 when you refer friends who sign up for the service.