Just came across this topic via a google search, so thought I'd comment further for any future readers in Wisconsin. I am a CPA in Wisconsin.
Yes, the OP would needs to obtain a seller's permit thru the Wisconsin Department of Revenue, pay WI sales tax, and file sales tax returns. The frequency of payment and filing will depend on your amount of revenue. The WI Dept of Revenue will tell you how often to file when the seller's permit is applied for (e.g., annually, quarterly, or monthly)... probably quarterly to start.
However, for some on here with gross revenue less than $1000 per year, note that you would be exempt from charging or collecting sales tax as an "occasional" service provider. Exempt = you don't have to obtain a seller's permit or collect sales tax.
Quickbooks is a nice program, but not necessary unless business grows well beyond what the OP is doing per year.
Finally, it sounds like the OP probably ended up slightly overpaying in taxes if he decided to come clean for past income collected that year. He mentioned paying the taxes out of pocket. If he had $10,000 in income, then he probably paid $10,000 X 5.5% = $550 in sales tax because he thought he should have collected that amount. However, he would have saved a few bucks in a legit way by reporting $9479 in sales + $521 in sales tax = $10,000 collected. This would have saved him in income taxes at year end too.
Obviously I recommend always discussing your business questions with your CPA to ensure the advice works for you specifically.
I hope this helps.
Visit us at www.halvcpa.com