There is the old adage that you get what you pay for, and the Internet is often no different. To have a web site, you need a couple of things.
First, you need a domain name. If the name of your restaurant is "Rick's Barbecue Stand," you need to register your domain name. One of the cheapest around is
www.godaddy.com - you may have seen their bizarre ads during the Super Bowl, but this is what they do. You may need to try a few variations of
www.ricksbbqstand.com or
www.ricks.com or
www.ricksbbq.com before you find one that works. Godaddy will let you know if you chose a domain that someone else already owns and suggest similar ones for you to use. This is important because it becomes your online identity and you want something easy to remember and something that would come up easy in searches.
Actually, that is the easy part. Because now once you have your domain, you need to pay someone else to "host" your site. Godaddy will also do this, but there are a few other places online, too.
Still...this is the easy work. The hard part is designing a good web site that gets your message across and is visually appealing to someone who stumbles by. Unless you have experience in writing computer code, your best bet is to hire this part out. This is where the bulk of your expense will be, but it will also have the greatest impact on whether or not your site is successful. Your best bet? Put up a notice at the local university asking for web design help. College students are always looking to make a few extra bucks, their knowledge of technology is current, and they will likely work a little cheaper than their professional counterparts. The more complex the work, the more money it will cost.
After that, you are ready to go...good luck!