Now you're ready to begin. Remember, keep an open mind when it comes to spellings and locations.
First, I'll briefly mention searching the federal census records on microfilm. Luckily I have not yet had to sit down in front of a microfilm reader. I find the online databases much more convenient. When searching for census records on microfilm you need to have a street address. Once you have that, you can use the ED (enumeration district) tools at stevemorse.org to find out which ED includes the cross streets nearest to that street address. These tools will then direct you to the rolls of microfilm to search. You'll have to go through the rolls page by page until you find the street address you are looking for. This can be time consuming and, if your ancestors complicated matters by moving before the census was taken, you may not even have the correct microfilm role.
Ancestry.com has all of the federal census images 1790-1930 online and all have a searchable database. For all years, you can search by head of household. Some years also allow you to search by any name. This allows for more flexibility in your searches. Ancestry.com has monthly, quarterly, an annual subscriptions that allow access to these records. Personally I prefer the annual subscription. It has the lowest cost per month (it is billed annually) and I don't have to deal with renewing or a lapse in access. At some libraries you can get free access to Ancestry.com. I like the freedom of doing a search whenever I want to. Sometimes I'll be doing something unrelated to my family research and out of the blue I'll get a new idea on how to find an illusive ancestor. If you don't want to spend the money on the annual subscription, organize you research so you have a list to go through after purchasing a monthly subscription. Or ask me for help.
So, where to start? First, either choose a specific year or just use the general search boxes on the first page. Always start with the exact spelling of the name of the person you are looking for and the state they lived in. You might get lucky and that's exactly how the name appeared on the census with good handwriting. Once you get into the search results, you can choose a specific census year and advanced search options will appear so, if you have a lot of search results, you can try to narrow down the list. Entering too much information in the search boxes will limit your search results. If there's any variation in any of the information you provided, you won't find your ancestor. If you've found the correct image, you can print it for your records. Make sure the option to print the "entire image" is selected. Also, change your printer setting to landscape so the image prints larger than it would with the portrait setting. You can also save the image to your computer.
If you weren't lucky enough to find your ancestor with an exact search, I recommend keeping the same name you entered but eliminate the selection you made for the state. I found several immigrant ancestors who ended up living in New Jersey before moving to Brooklyn. Before discovering this, I thought they had all settled in Manhattan first, then Brooklyn. That explains why I couldn't find naturalization papers in NY for some people...they were naturalized in NJ.
If you still haven't found the people you're looking for, you'll need to use the "soundex" search option. I recommend selecting a state again before you do this because you will have a lot more search results to go through. Some of the search results you get will obviously not be matches but, again, keep an open mind. I finally located one branch of my family under the name Berlin instead of Berland. If you're ancestors were immigrants, imagine what the name would sound like with a heavy accent. If you're looking for a complex spelling of a name, keep in mind that a new arrival to the US may not have been able to spell the name for the enumerator. A very long name could have been shortened. The first letter could have been changed to one that sounds the same in English (my Jablon ancestors changed their name to Yablon in the US and UK...the "J" in eastern Europe is the same as the "Y" sound in English). Sometimes you need to look at the entire family grouping to know that you have the right people. Remember nicknames...sometimes a person named John went by Jack. My great grand uncle C. Israel Lutzky shows up on the Ancestry database as Sarall Lutzky. The transcriber didn't see the C as an initial. Luckily I knew his wife's name.
Some ancestors will remain illusive. I can't find a single record for the Rotheim branch of my family. I know they're there. It is possible that some census images are lost or missing but it's odd that I can't find them for 1910, 1920, and 1930. The name must be so badly misspelled that even the soundex can't catch it. I've even tried searching by given name, no surname, and still can't find them. When this happens, I step back for a while, even for months or years, and then try again. Start your new search fresh. Don't say "I already looked at that record so I know it's not them". Also, if you had bad luck and the data from that census image was missing the first time you tried, it might be there now.
If you do find an error in the Ancestry database, submit a correction or comment to them. They won't change the original record but once these corrections and comments make it into the database, they will come up in the search results.