You have a few options here:
Register for a post office box by clicking here (https://reg.usps.com/register?app=POBO&appURL=https://poboxes.usps.com:443/poboxonline%2Fpages%2FmyAccount.do). Alternatively, you can fill out and take the paper form (https://about.usps.com/forms/ps1093.pdf) to your local post office.
Use a coworking space. This is probably the most expensive option, but it includes more than just a mailing address. A coworking space is usually full of entrepreneurs, which could end up being a valuable networking and learning opportunity for you.
There are a plethora of online business management companies who offer mailing address services.
Hire an attorney during the formation process. An attorney may use his or her own address as your business address on formation forms.
LawTrades (www.lawtrades.com) is a site created by entrepreneurs for entrepreneurs, where our mission to help startups find affordable legal services. Just tell it what you need, and it'll introduce you a handful of vetted startup lawyers. Then, compare by price/reviews/professional chemistry and simply hire the best lawyer.
This happened to me a few years back. I had a few options.
1. Easy - Get a post office box at a local UPS Store or the like. That way you are separated from your business.
2. Harder - Sign up for a co-working space. You get a desk and wifi and usually conference rooms. It is a great way to work and share in the space but have a separate way to work. They are usually fairly inexpensive and allow you to get out of the house and work with others.
There are also services like http://www.intelligentoffice.com/ that allow you to use their address as your professional address. They also provide secretarial services such as phone answering. I found their prices to be a bit expensive but for some companies a professional address is more important than for others.
I am going to assume you are working from home since you are asking this question...Your business address can be anywhere that your business can receive mail. So, if you have an office, use that. If you have a friend with an office, ask if you can get your mail sent there. A more legit solution may be to get a Virtual Office at one of the many shared spaces popping up around your town. Usually, these co-working spaces will have a "virtual office" option whereby you can pay a monthly fee and use their address for your business, get your mail forwarded, and get discounts on conference space.