First and foremost, you really need to pick a good domain name. While the default web address you get from us can work in the long run, you will be best served using a custom domain name. There are basic instructions for how to set these up in the help section.
The tricky thing about domain names these days is that most of the best have already been taken. You will need to find a balance between shortness, memorability, and search engine optimization. Generally speaking, a shorter .com domain will always be the best for you and your customers. An example of this would be Alfonsos.com. This sort of name looks more professional and is easier to type out (which is a big bonus for mobile users).
Often times, your first-choice domain will already have been taken. In this case, you may want to try spelling out more of it (AlfonsosHotDogs.com or AlfonsosHotDogShack.com) or trying an alternate extension (Alfonsos.net). Alternate domain extensions like .net shouldn't affect your rankings very much, but may be more difficult for customers to remember.
Another option is to add your city or location to the domain name. This is actually very good for search engine optimization and is highly recommended. If you use a domain name like AlfonsosHotDogsPortland.com, you will get a boost for all the keywords in your domain name, in this case "Alfonsos", "Hot Dogs", and "Portland". In this way, you can actually be better off than with a domain name like Alfonsos.com, where you will only get a boost for your restaurant's name. A name like "AlfonsosHotDogsPortland.com" can do very well in searches for terms like "hot dogs portland".
Please keep in mind that one thing the search engines weigh quite a bit is how long your website (with its current domain) has been on the internet. If you have a domain that you've been using for years, you really shouldn't change it to something new unless your domain is inaccurate (for instance, if you changed your restaurant's name). In other words, changing your domain name for the sake of SEO is not a good idea unless the domain name is really bad.
One thing that should be mentioned (since a lot of people ask about this) is that having multiple domain names is not a good idea. Most search engines will punish you in some way and may possibly even delete your website from their results pages if you have multiple domains resolving to the same website, so don't do this unless you really know what you're doing. You've been warned!