One way that I've managed to keep all of these aspects under control is through a service called Outright. It automatically links to my Paypal account to track all purchases for my business and all sales that I make through Etsy. All I have to do is go back and add in the info from transactions from my bank account, and it will calculate profit and loss graphs, and plenty of other useful tools. If you have a web-based small business, I'd definitely recommend checking it out.
Something that I always mention when people ask about starting a business is keeping it legal. Maybe you're not planning on this becoming your day job, but the IRS will definitely make sure they get all of the money they're owed and then some, so it's a lot safer to get the legal aspects going from the start of your business. Now I can only speak for a small portion of Missouri specifically, but most states will require to you register for state sales tax and a retail license. Your business name must be registered if it's not your exact legal name (this is called a DBA, doing business as) My city and county also require registration.
There's about a million other things I could go on and on about (taxes for instance) but those are the best places to start if you're trying to get a business going. If ever in question, the best place to reference is always irs.gov, or your state's .gov website for small businesses.