This year is a census year in the US. The census happens every ten years and is a massive undertaking by the federal government to count all its residents. The information is used in a number of ways, including determining how funds are distributed and whether the number of representatives in the House needs to be adjusted because of population losses or gains. Names and personal information are not released until something like 70 years have passed (for example, the 1940 census raw data won't be made public until 2012), but how can you get to the aggregate data? Check out census.gov.
If you want data for the years since the last census, check out American FactFinder in the menu on the left. Not every year is represented, but at least the information is more recent than the previous census.
To get local statistics, use the Population Finder on the right side of the homepage. Also check the QuickFacts, also on the right. Area profiles include states, counties, and towns of at least 25,000 people. To find a smaller town, select the state the look in the "city" menu. The last option is "Other places not listed," and you might be able to find your town by searching that way.
What if you need data from other years? Take a look at the Statistical Abstract of the United States. The information is housed on the Census website, but the information is gathered through means other than the census questionnaire and is released yearly. Generally you can't get down to town level, just counties or cities. Click the "Earlier Editions" tab to get the Stats Abstract back to 1878. Other historical data is available back 1789. Also check the links on the left to get to specific sections of the current edition. Other similar resources, such as the "County and City Data Book," are listed on the right of the main Stats Abstract page. Your local library may have some of these books in its reference collection, but the information is also all on the Internet. Feel free to ask your local librarian if you need assistance with the books or the website.
ePodunk. Type your town into the search box on the right. You should get a long page with a list of sections on the left. Under each section are links specific to your location. You can find everything from population to cemeteries to real estate to local events. This might be a great starting place if you're moving and are curious about your new town, or if you need genealogical resources.