First specify the surname, name and patronymic of your most distant ancestor. It will begin the search for all the other family, because I need a lead. In addition to these data we should know the year and place of birth of the person whose information you are going to look. After all the data about the people in archives are grouped not alphabetically, but by years and districts. So the priests were parish registers.
Next you need to contact the archive at the place of birth of your ancestor with the request. The request can be sent personally by mail. Archive staff will look for records about you to the right person in the registers. There may be mentioned not only the date of his birth, but also parents. So you can find the information for the further restoration of the pedigree. Because the records about the parents can be specified, where they are, which belonged to the estate.
If your ancestors belonged to the merchants, the Cossacks, prospectors, etc. classes, you can look for information about them in the historic funds or the regional Museum. But it will have to do yourself, diligently working in the reading room. But I can be receipts, petitions of their ancestors , and similar information.
The discrepancy between the actual and the expected date of birth of your ancestor should not be more than 5 years.
Some state archives have special departments dedicated to locating the ancestors and making a family tree. But this service is worth a lot.